The Avengers: Ultron. The Avenger's most iconic nemesis aside from maybe Kang the Conqueror, he is the ultimate AI in the Marvel Universe created by Hank Pym to be a benevolent force, but of course A.I. Is a Crapshoot so it didn't turn out that way and he became a completepsychopath who despises humanity and relishes in causing as much pain and suffering to humanity (particularly the Avengers and his creator) as possible. Not only is he virtually indestructible, able to regenerate from the rare moments he IS injured, but he's also got a plethora of abilities (i.e. energy beams strong enough to destroy planets, flight, control over magnetism and the ability to hack and control all manner of technology). He's also DANGEROUSLYintelligent, his mind adapted from the human brain (his creator's to be precise), meaning he can learn, adapt and store information in a way no other human or machine can. He's so dangerous and loathed that Doctor Doom himself has stated that Ultron is the scariest thing to have ever existed. Yeah. Considering that he once hijacked the Phalanx and took over the entire Kree galaxy in a matter of hours, this is entirely understandable. No doubt everyone, including The Avengers, rue Hank Pym for the day he was created (which may actually explain his less than stellar reputation in-universe...). Ultron is also a source of personal horror for Hank Pym because his AI is based on Hank's own brain patterns. Hank doesn't like what that implies about himself.
Captain America: Nobody likes Red Skull. Nobody. Like the The Joker from the DCU, dropping his name in casual conversation in the Marvel universe underworld will cause everyone to go quiet and stare at you. Doctor Doom and Magneto have both tried to kill him... not because he got in their way, but just to get rid of him.note Though Doctor Doom is Roma and Magneto is Jewish, so they've got a legit reason to hate Red Skull When he briefly disappeared (thanks to Magneto's attempt on his life), his return caused Captain America to have a temporary Heroic BSoD.
"All these months I've lived in a fool's paradise, refusing to believe his claim to be the real Red Skull, refusing to believe that my greatest enemy had found a way to cheat death...but he has. The Red Skull lives- God help us all."
Notably, in a rare DC/Marvel crossover story, the Red Skull teamed up with The Joker to enact a villainous scheme. Their alliance lasted up until the point when the Joker realized Red Skull was indeed a real Nazi, and he wasn't just wearing a swastika for kicks. At that point the Joker pulled a gun, and officially dissolved their partnership. That's right, kids. Even the Joker refuses to associate with the Red Skull.
Captain Britain: The Fury is such a ruthlessly effective Hero Killer it makes the Sentinels look like Mecha Mooks. When Mad Jim Jaspers was at the height of his power, to the extent that Merlin didn't think even destroying the universe would be enough to stop him, The Fury showed up and killed him. For pretty much any hero who has encountered it and lived to tell the tale, meeting it again is the absolute last thing they want to do.
Daredevil's reputation is only slightly less scary than that of The Punisher. Case and point, when his identity is revealed by the media, only the C-lister Mr. Hyde tries to take advantage of this moment of weakness to attack him, and later Stilt-Man says the whole criminal underworld is too terrified of him to attack. This claim is also supported by Ben Urich. Later, Matt declares himself the new Kingpinwhile unmasked, and not a single person takes advantage of it. They just flee.
Elektra's reputation and talents as the world's deadliest assassin have put her well into this category. Wolverine admitted that she is one of the only people he actually fears. SHIELD agents are usually given protocols in the event that they encounter a hero or villain going on a rampage. In the event that they encounter Elektra their instructions are to immediately cease whatever assignment they are on, get the hell out of there, and pray she doesn't decide to go after them.
Always a Bigger Fish; the demonic living superweapon Zom scares the hell out of Dormammu's sister, Umar (who's roughly his equal in power), and took the combined efforts of Dormammu and Eternity (along with unnamed others) to seal in a can. Imagine how Dormammu must feel about Zom if he was willing to work with Eternity to stop it.
Bizarrely enough, Shuma Gorath is a lesser example of this compared to Dormammu and Zom despite being a Lovecraftian horror more powerful than Dormammu and on par with Zom (if not stronger). This is due to Shuma Gorath's Charm Person powers that make everyone it rules over revere it. Everyone not under its influence and aware of its existence fear it.
Fantastic Four. There is barely a single supervillain out there who hasn't at the very least heard horror stories about their terrifying badassery and feats of awesome. Like the Doctor over in this trope's Live-Action TV subpage, they have entire races of aliens, most of whom they don't even remember meeting, who tell tales about them as mythical bogeymen to scare their children with. Case in point:
Reed: It turns out they speak trinary code. So I said to them, "I am Reed Richards of the Fantastic Four, and I..." Sue: Go on... Reed: Actually, that's as far as I got. It was enough to send them running.
Even among people who've never actually met the Four, the prospect of facing people who defeatedGalactus (more than once!) would legitimately be terrifying.
The team has developed a mythic reputation on some planets who have tried to invade Earth, only to be challenged by them and have their asses kicked; the cultures of these planets think of them in the same way Earth thinks of The Four Horsemen of the Apocalypse.
Sue Storm is often depicted as the most dangerous member of the team due to her versatile powers and vicious temper when provoked. The Black Cat once stated that Sue is more terrifying than any of the experiments, weapons and security measures in the Baxter Building, and specifically waited until Sue was absent to carry out a plan to steal a book from Reed's collection.
Galactus, the devourer of worlds, counts for self-explaining reasons, though he can be reasoned with. Unlike his counterpart from Earth-1610, who kills organic life just because it is disgusted by it... and then 616 Galactus ended up in the Ultimate Universe and promptly made that version of Galactus his new Herald.
The Incredible Hulk: The Hulk scares the hell out of the rest of the Marvel Universe. The most powerful heroes - even knowing full well that he's Not Evil, Just Misunderstood - take his arrival as seriously as that of someone like Doctor Doom, if not more. Given what a rampaging Hulk can do, it's not without reason. He could get angry, you see. And you wouldn't like him when he's angry. In everything from trading cards to the actual comic books, other heroes are shown to be very reluctant to confront him.
In Greg Pak's run on Hulk (1999), people start to wonder if Banner is actually the more dangerous one. He is.
Deadpool had a memorable meltdown when he found out that he had to get a blood sample from the big green galoot in an attempt to cure his currently-failing Healing Factor. His reaction was to talk him up using the lyrics of his old cartoon theme song ("Ain't he unglamorous").
Subverted with Captain America. He not only holds immense respect for Hulk in either form (he bitched out the Illuminati for sending Hulk into space) but it has been said by numerous characters that the only thing Cap would need to do to stop a rampaging Hulk would be to stand in front of him.
The new Hulk personality introduced in Immortal Hulk terrifies even Thor, he refers to him as a Devil.
Iron Man: Tony Stark is now this to the Dark Elves of Svartalfheim, who are basically The Fair Folk on crack, with even Malekith the Accursed, an incredibly powerful psychopathic sorcerer who fights and actively trollsThor, being wary of him at the very least. Malekith also comments, after Tony, in a Cold Iron armour, tears his way through Svartalfheim hunting for the rings of the Mandarin (which Malekith had acquired by murdering the former owners), that the upshot of it is that they now have a new story to terrify the children with.
The Mighty Thor: Thor. His buddies in The Avengers count some of the nastiest, most powerful beings in the universe as enemies, but even guys like Ultron, Count Nefaria and Thanos become seriously concerned, step up their game, or maybe even flee when the God of Thunder arrives on the scene. And don't even start with the lower-tier villains. Captain America once bluffed King Cobra of the Serpent Society by saying that Thor was outside Cobra's hideout, and that he was losing his patience. Cobra got the hell out of dodge.
His reputation is even worse among the Giants of Jotunheim, his ancestral enemies. He's basically the Bogeyman to them, young and old alike.
"They speak my name in whispers in the nurseries in thine icy home!"
The Punisher: The Punisher is this, foreveryreasonyouthink. The very mention of the name "Frank Castle" is enough to scare the bejeezus out of the toughest, meanest thugs, killers, and mafiosi.
At one point in the MAX series, a hardened mercenary from the Balkans drops to his knees and starts sobbing and praying at the mere sight of the white skull.
This goes as far back as his first comic, Circle of Blood, where at the start Frank is incarcerated in Riker's Island for the third time, and most of its population are afraid for their lives.
In one arc, a wannabe tough guy mafioso hits on a brilliant plan to take out Frank: make him angry (by digging up his family's bones and pissing on them) so he'll make stupid mistakes. Part one works, part two doesn't—he starts wiping out entire gangs at once and leaving a single survivor to tell the police they need to rebury his family's bodies. While Frank is rampaging around the city hitting every criminal outfit he can think of, two small-time thieves considering breaking into a jewelry store debate whether they're still too small for Frank to consider killing, and wisely run away.
In a Punisher / Batman crossover, the Joker had an Oh, Crap! moment on realizing Frank really was going to put a bullet in his head. Batman intervened at the last second, of course.
Wilson Fisk aka Kingpin is a ruthless crime lord and powerful mob boss whose empire spans pretty much all of New York and possibly the world as a whole. Nobody crosses him without good reason and many heroes (and villains) are cautious when dealing with him, less because of what he'll do to them physically (though he's easily capable of kicking your ass if you piss him off) but what he'll do to their lives. He's a very well-connected man and can ruin the life of anyone who sticks their nose in too deep, not to mention his superior intelligence and strategic mind. His oldest enemies (Daredevil, Spider-Man, and the Punisher) all keep cautious when dealing with him in many cases.
Venom, the Evil Counterpart to Spider-Man, was widely feared in the supervillain community in his heyday. Which makes sense when you think about it: If Spidey's foes get beaten by this one all the time, imagine how they would feel about a stronger and deadlier version of Spider-Man who is perfectly willing to kill. Though the fear subsides somewhat once they realize just how severe Venom's vulnerability to fire is.
The significantly more unhinged Carnage doesn't have the fire weakness (or at least nowhere near as bad as his daddy does) and the villain community does their best to stay the hell away from Carnage should they value their body remaining in one piece. In fact, it's shown in the comics a number of times that as brave as Spidey is (not to mention the sheer number of enemies he's dealt with all at once in other stories), Carnage is the ONE villain he refuses to face alone. Spider-Man and Venom even team up to take him down; bitter as their own rivalry may be, they both agree Carnage is as dangerous as he is unhinged and needs to go down before he causes anymore chaos than he probably has already.
Spider-Man: (thinking to himself as he's desperately pummeling Carnage with everything he's got) If I'm going to take him down, I've got to do it fast. Keep hitting him and hitting him till he falls. Because if I give him so much as an instant to breathe... everyone in this room... maybe in the whole building... is as good as dead!
The symbiotes have this reputation throughout the galaxy, as Flash Thompson learns when he joins the Guardians of the Galaxy.
Spidey himself becomes this when he's being quiet: when the Motor Mouth stops running, his enemies conclude that the Friendly Neighbourhood Spider-Man is done being friendly and is about to stop pulling his punches (and the dude can hoist a damn train car over his head like it's nothing). Memorably subverted in The Quiet Room in Amazing Spider-Man Annual (2014) that went memetic; his villains are desperately trying to get him to crack a joke, conclude that since he isn't they're totally screwed and surrender... and when he's handing them over to the cops afterwards, Spidey manages to gasp out that he has laryngitis. And there's that time when Spidey beat Otto Octavius so badly that Doc ended having to go to therapy for a while due a pretty serious arachnophobia he suffered as a result. Titania's another that Spidey's beaten so badly that she becomes afraid of him for years afterwards.
The Rhino is one of Spider-Man's most dangerous adversaries and occasionally gets this treatment. While he's not as methodical as Green Goblin or Doctor Octopus, he more than makes up for it with his sheer overwhelming strength. In one run, Peter admitted to being afraid of the villain and admitting that his victories over Rhino had either come from preparation or sheer luck. Considering the character is also known to trade blows with the Hulk, he's earned his threat level.
In Sub-Mariner: The Depths, Namor is considered to be the most terrifying thing in the ocean, was blamed for the sinking of the Titanic, and apparently kills anyone searching for Atlantis. And he will track you, then murder your crew. He kills two submarine crews and the crew of an entire research station. But he ultimately spares Dr. Stein, so Stein can go on to debunk the myth of Atlantis for the public like he always does, allowing Namor and his people to live in peace. How Namor got this information is unknown.
Thanos: Thanos the Mad Titan is a name/title that not only gains attention from every last hero on Earth when dropped but also from the likes of Galactus. He is completely unpredictable, and every time he is scheming on another plan, you can be sure a catastrophe is about to follow.
Ultimate Fantastic Four: Reed Richards goes through a Face–Heel Turn and effectively becomes Ultimate Doctor Doom complete with facial scarring. The storyline covering this is titled "Ultimate Enemy" for good reason.
Ultimate X Men: Before joining the X-Men, Wolverine had this reputation. The mere mention of his name scared Scott and Jean.
The Unbeatable Squirrel Girl: Squirrel Girl has gotten this reputation among many supervillains and organizations, to the point where they refer to her as "The Anti-Life" and "The Slayer of all that Breathes". Dr. Doom just lets her do whatever she wants in Latveria, and Deadpool puts her in the same threat category as Iron Man and Thor.
Magneto. He's most of a century old, he's one of the most powerful beings on the planet, and even weakened, he can tear through most opposition like wet paper. One SHIELD Agent on a squad assigned to hunt him down in his solo series mutters after seeing the devastation that he thought Magneto was weakened, and one of his colleagues replies that he is - and he still managed to slaughter everyone in his way. He's frequently compared to a natural disaster, which leaves out the fact that he's near cosmically powerful, brilliant, and when faced with those who victimise mutants (or just his enemies in one of his less stable periods), inventively sadistic.
Mister Sinister. The long-running Arch-Enemy of the X-Men as a whole (though Scott Summers and the Summer bloodline in general have his particular mutual ire). Don't let the mildly campy name fool you. He's one of the most dangerous, feared and loathed characters in all of Marvel comics, especially in the mutant community. Originally a British man from Victorian London named Nathaniel Essex, he is cruel and abhorrent Mad Scientist who became fascinated (or rather obsessed) with Darwin's theory of evolution and saw mutants as the key to humanity's next step. To that end, he began experimenting on mutants (and himself) and over the next two hundred years devolved into an Evilutionary Biologist completely devoid of conscience and a sadistic streak to rival the likes of Dr. Zander Rice (Apocalypse's... modifications of his body also made him creepier by making his outward appearance reflect the monster he is on the inside). Any time his name comes up in an X-Men comic, you know that it's going to be an extremely traumatic and painful experience for any mutant involved.
Scott Summers developed this reputation in the Utopia era. Sure, before people didn't take him lightly - experienced superhero and leader with the will to corral Wolverine (who he could match in a fist-fight) and the raw power to obliterate that vast majority of opponents with a literal Death Glare. His reputation as a mutant revolutionary reached such a point that even a psychic projection of him was enough to rally mutants worldwide in the run-up to Inhumans vs. X-Men. Even his temporally displaced teenage self has shades of it, after young Jean is kidnapped by the Shi'ar, squaring up to Gladiator and saying, "give me Jean Grey or I will kill you." Gladiator, having said that the older him would know better than to push him, corrects himself and says that he sounds exactly like his older self, with no doubt that he would follow through if he could.
Jean Grey, mostly because of her time as host of the Phoenix Force. She was so feared by the Shi'ar that even well after her death, they were willing to wipe out her entire family to try and ensure the Phoenix didn't return to haunt them, and kidnapped her teenage time-travelling self - something which, one of the Imperial Guard warned Gladiator, could very well have just triggered the nightmare they so feared. Also, while she's the Nice Girl and Team Mom of the X-Men, even if the Phoenix isn't involved, she's treated with wary respect. Considering she's one of the most powerful mutants of all time, and mother of most of the rest, this isn't surprising.
Cable, to the point where it's explicitly stated in the Krakoa era, when he's compared to his younger self, "everyone was scared of the old man." During his 'Saviour Cable' period, running Providence and at the height of his powers, he scared everyone so much that SHIELD repeatedly tried to displace him, with Nick Fury saying dourly that they'd have to wait and see if he made a mistake. Not when - if. Eventually, when sending the X-Men and a global military assault both failed, they had to resort to calling in the freaking Silver Surfer.
Nate Grey, Cable's baby brother, was this from the start, given that his raw power matched that of the Dark Phoenix, and aside possibly from Franklin Richards (who he got on very well with) he was possibly the single most powerful mutant in existence. Add a volcanic temper, a lot of (understandable) paranoia, and an unpredictable power set which warped realityin his sleep, and you can understand why everyone was so wary of him. It only got worse when he stabilised his powers during the Shaman era, becoming Crazy Sane by his own admittance and developing a rather terminal approach to justice. His return in Dark X-Men had Dark Beast, who was happy snarking at Mystique and Norman Osborn, ready to run screaming. While he wanted to make the world better, Dark X-Men noted that an Omega-class mutant with a social conscience can be quite unnerving. It only gets worse in the run-up to, and aftermath of, Age of X-Man, traumatising a lot of very powerful mutants - and that was when he was trying to help them.
Rachel Summers' reputation is an extension of her mother's, for the most part, thanks to in-depth connection to the Phoenix. However, she also expands the reputation on her vengeful rampage in Shi'ar space after her family was massacred. Likewise, she also had one in her own time, when she was Brainwashed and Crazy, as one of the most powerful mutant Hounds.
Sabretooth is a sadistic, completely unhinged psychopath who treats everyone not allied with him as prey to be hunted down and slaughtered. This would be enough to make a lot of people nervous, but the worst part is that the bastard is smart. Not only is he a centuries-old professional soldier/mercenary who has been trained by countless agencies and fought in innumerable wars, but he's an exceptionally adept manipulator who can play almost anyone like a puppet without their even knowing. Just mentioning his name or the fact that you hired him is enough to cause alarm in a good portion of the MU.
Wolverine got similar treatment in one storyline when the Hand resurrected and brainwashed him into being an agent of HYDRA. This makes perfect sense, since evil Wolverine essentially is Sabretooth.
The Sentinels are this to the whole of mutant society, especially after the Genosha Massacre. They are the ultimate embodiment of senseless racism and cruel genocide in the Marvel Universe. This causes problems for Juston, in-universe and out, since his Robot Buddy is a Sentinel.
On a more humorous note, the tiny alien dragon Lockheed is this for the Brood.
Humanity in general has become this. For one of the earliest examples, one story had an alien invasion fleet posed to attack what they saw as an Insignificant Blue Planet that would quickly fall to their might. That is, until one of the people on the bridge informs the leader that Earth has managed to turn away Galactus multiple times. They proceed to immediately run like hell.
