Foxy. Once he leaves Pirate Cove, you may not have long to live. If you see him sprinting down the hall through camera 2A, it may already be too late. If you succeed in blocking him off, his banging on your door will drain precious power.
More so with Freddy. Unlike Chica and Bonnie, he doesn't stand at either doorway or window with a chance to shut the door in his face. While he won't enter your office as long as the cameras are down, and he can be held back by viewing him on the East Hall Corner right outside the door, he moves inside the instant any other camera is viewed and his door is not shut. Once he reaches your room, he waits for a random moment to attack, emitting an eerie whisper. Once he does attack, it's game over. In Help Wanted's recreation of FNAF 1, Freddy can now stand outside the right door, but otherwise still remains as dangerous as he did in the original game.
A good number of players have died a lot to The Puppet, the danger that the Phone Guy advises you to wind up the music box to avoid. What makes it particularly dangerous is that you can often be in situations where an animatronic is peering into the room, but you need to pull the camera up to wind the music box, which requires you to not be wearing the mask, your line of defense against most of the animatronics.
The original animatronics are much less forgiving than their Toy counterparts. The best way, it seems, to avoid them killing you is putting on the mask before they start scanning the room, which usually is only a split second. Bonnie in particular is difficult for this reason.
BB and Foxy seem to love the horrible combination that is them teaming up. BB disables your flashlight, the line of defense which Foxy can only be stopped with.
Phantom Puppet. He leaves you completely defenseless for about 15 seconds, during which Springtrap can make a ton of progress through the building or climb through a vent unobstructed. And as if that wasn't enough, he disables your ventilation and your audio, resulting in a grand total of about 21 to 24 seconds where you can't do anything against Springtrap. On Night 5 and Night 6, this is more or less fatal.
The glitch plane enemies, which are so powerful they could even give level 30 characters a run for their money. _>>>>>> (a hybrid of Boxbyte and Graveweed) is probably the worst, as it spams Alarm, an extremely powerful attack that is used by the Disc-One Final Boss.
The Prototypes from Blacktomb Yard use Hot Cheese rather continuously as a way to drain the party members' health.
Funtime Foxy, and to a lesser extent, Toy Freddy, Foxy and Rockstar Bonnie. During attempts at 40/20 to 50/20, streamers found out multiple Easy Level Tricks, one of which is the plushie trick wherein leaving the camera on CAM 2 stopped Baby, Nightmare Bonnie and Nightmare Mangle, fellow That One Bosses, completely. The catch, however, is that you have to leave the camera on CAM 2 without switching cameras, as doing so immediately triggers the plush animatronics to attack you. Enter Funtime Foxy and Co., whose mechanics required switching cameras in order to stop them. While Toy Freddy and Foxy can be sufficiently ignored given enough luck, you have to switch to Funtime Foxy during his showtime, and you have to find Rockstar Bonnie's guitar (though you can prevent him from appearing entirely by not raising the monitor at certain times). The only real saving grace here is that there's a few seconds where you can switch cameras while the plushies switch and prepare to attack, allowing you to stop the other animatronics. There's also the Death Coin, which can be used on one of said "monitor" animatronics. More info can be found here.
Nightmarionne's mechanic is that he appears slowly over your cursor, and then fades in if you continue to hover over him. Due to how big his mug is and how quickly he fades in, it's very difficult to manage him, and he can sometimes kill you in one second. And if he's covering an important button, like Rockstar Freddy's payment button, or anywhere you need to shine your flashlight on, he can easily get you killed without even killing you himself. Doesn't help that he fades out much slower than he fades in. While he's somewhat easy to manage for expert players, there's rarely a 50/20 run where he doesn't end up being a major nuisance.
Nightmare BB's mechanic sounds simple, yet he manages to be a real problem. If you lower the cameras and he's standing, flash him with your flashlight to get him to sit down. Sounds easy, but when characters like Trash and the Gang, Helpy, and if you're unlucky with Dee Dee, the Minireenas, block your view, it can be hard to notice whenever his relatively small figure is standing up. But that's not the worst part. If he's sitting down and you flash your light on him for even a split second, he'll kill you. Trying to flash away the Freddles or Phantom Freddy and your mouse slips a bit to the right? Get ready to count his teeth. Even Nightmare BB knows how much of a bastard he is to deal with:
The Glamrock Endoskeletons. While their gimmick of only being able to move when not seen isn't the worst aspect of them, what makes them dangerous is the fact that they are quite fast when unseen, often come in absurdly large groups, and can easily surround an unprepared player. To make matters worse, they also lack a lot of the loud audio cues to signify their presence like the other animatronics, save for an unsettling (and quiet) music motif that plays when they're active in an area. They return in the Ruin DLC, working just the same as before, with however many that Cassie encounters being randomized per playthrough, but unlike the base game where they showed up in several places, they only roam the Superstar Daycare, and Cassie can use the Security Mask to try to avoid them.
