Sonic making his introduction in City Escape, the first stage. After being falsely accused of crimes committed by Shadow and accordingly captured by G.U.N., he bursts out of one of their helicopter gunships and escapes with ease — riding a scrap of metal torn off one of the aircraft's hard points like a snowboard and performing various tricks on the way down.
Sonic: Talk about low-budget flights! No food or movies? I'm outta here! I like running better.
City Escape in general is this as a first level. It's about as linear as linear gets, but it teaches you just about everything you need to know for the adventure going forward, including being rewarded for good platforming. And that's on top of the fact that it's memorable for its atmosphere as G.U.N. is pursuing Sonic nonstop, going so far as to sic a giant truck on him while he's running downhill. There's a reason this is one of the most iconic levels in Sonic's long history.
Shadow's whole introduction and basically any scene with him — one of few characters to steal the spotlight from the title character.
On top of that, his introduction is the first time we see the iconic Chaos Control in action. And Sonic gets one for figuring out the technique in just a few short seconds.
This game is the first to give us two Super Hedgehogs.
This cutscene. It perfectly displays the character relationship between Sonic and Shadow and sets a mood that if Shadow isn't stopped here, Eggman will take over the world — when played from the Heroes' side, anyway. The music just makes it all the more epic.
Sonic: Alright, Shadow. Time for me to finish this! Shadow: Before this is over... I'll show you the true power of CHAOS CONTROL!
As discussed on the Fridge Brilliance page, it's likely that Sonic won this fight: the Hero Side Ending shows Sonic stopping the Eclipse Cannon from firing, while the Dark Side Ending shows the Eclipse Cannon overloading with power. The theory goes that Sonic caused the overload that occurs in the Dark Side Ending by planting the Fake Emerald into the weapon. Furthermore, if the Dark Side Story were canon, Shadow would most likely have killed Sonic after defeating him. Therefore, Sonic canonically defeated the ultimate lifeform in one-on-one combat. If you read some of Shadow's feats in this game, you'll know how impressive that is. Not bad, little hedgehog, not bad...
After organising Shadow and Rouge to raid and detonate an entire island, and with six of the seven Chaos Emeralds in his possession, he declares himself ruler of the Eggman Empire, announces to the entire world that they will surrender or die, and, just to prove he isn't fucking around, powers up the Eclipse Cannon and blows up half of the moon.
The entire "Fake Emerald" scene. Successfully holding Amy Rose at gunpoint aside, he knew all along that Tails had prepared a fake emerald, and pretty much tricked him into admitting that it was the one they were about to hand him in exchange for Amy's safe delivery. And for this little trick, he traps Sonic in an explosive capsule and launches him out of the ARK. Worse still, the capsule's explosion, and Amy's grieving are shown onscreen in the Dark Side story.
Even after the scuffle with Tails that follows this, he still manages to get his hands on the real emerald.
Sonic performs Chaos Control for the first time, with the fake emerald, thus escaping certain death by one of the narrowest margins in a Sonic game. Even Shadow was impressed when he learned about it.
Shortly thereafter, despite being winded from the effort, Sonic confidently blasts once again off to stop the Eclipse Cannon from firing at all costs.
Sonic: I'm worried about Tails and Amy. I hate to ask, but could you [Knuckles] help them? This is our last chance. I got it: before the Cannon fires, I'll slam dunk it in there!
The first level of the Hero story, City Escape. It's not everyday you see someone freefall out of a helicopter, board down the side of San Francisco with a part of the wing, and eventually outrun an enormous military truck.
Heck, what about Eggman's first level? The Dark story opens with him charging Prison Island, guns blazing and his theme triumphantly flaring.
What about the encounter in the jungle? "I found you, FAKER!" Heck, Shadow telling the Sonic he's the faker, and then telling him that he's not even good enough to be his fake, while Sonic answers back with an epic "I'LL MAKE YOU EAT THOSE WORDS!" and, if you're playing as Shadow:
Shadow had a very overlooked moment of awesome in hindsight. He fell to Earth, from SPACE, and SURVIVED. Come on; ultimate lifeform or not, that's gotta count for something.
While everyone (including Eggman) is racing around trying to stop the Space Colony ARK from falling to Earth, Shadow just stares down at Earth, claiming that he doesn't need to help because it's useless anyway. After Amy gives him a speech about how, even though Humans Are Bastards, they can be good too, Shadow finally remembers Maria's true final words: "Give them a chance to be happy!" and quickly runs off to the cannon's core. Meanwhile, Sonic and Knuckles are preparing to do battle with The Biolizard, when suddenly they hear; "Leave this one to me." Shadow walks towards his prototype, calm determination etched into his face, while Sonic and Knuckles stare at him bewildered. Shadow whirls around, declares "I'll take care of this, while you run to get the Chaos Emeralds!", trashes The Biolizard in an epic one-on-one battle, and finishes with "GET OUT, YOU UGLY PROTOTYPE!".
Shadow also gets a pretty epic line when you switch to him in the Final Hazard boss fight:
Shadow:HERE I COME, YOU CREEP!
Eggman's display of cunning trickery when he confronts Sonic and Tails on the ARK with Amy hostage, completely destroying their plan based around the Decoy Emerald, is worthy of respect.
Eggman: (points a Hand Cannon at Amy's head) Let's take care of business first, shall we, Sonic? Hand over the Chaos Emerald, slowly, and back off. That is, if you really care for her...! Sonic: (aside to Tails) Handing over the Fake Emerald, I can kill two birds with one stone! Tails: (nods to Sonic) Eggman: Put the Emerald down, right there, and back off! Sonic: (sarcastically, as he approaches with the fake Emerald) You've turned into a big-time villain, Doctor! (Eggman chuckles to himself and presses a button as Sonic steps into the middle of the room, immediately sealing him inside an escape pod rigged with explosives) Sonic: Wha...?! Eggman: You thought you could trick me with that fake Emerald, didn't you? Tails: ...so, how did you know it wasn't the real one? Sonic: (cutting off) TAILS! Eggman: Because you just told me, fox boy! Tails: (Stunned Silence)
Amy: I know that people fight over the most trivial things. Some people may be selfish, like the professor said... but they're basically good, if they try their best and never give up on their wishes. They always have a reason to be happy; that's why you should help them out! Saving them is a good thing! Shadow, I beg you, please do it for them! Give them a chance!
Amy's entire involvement in this game is a crowning moment for her developmentally. In Sonic the Hedgehog CD, Amy was captured by Metal Sonic and was a prisoner the entire game. In Sonic Adventure, Amy spent most of her time running away from someone or something until she finally decided to stand on her own two feet and fight her own battles. She then decided to become a stronger person and teach Sonic to respect her and this game perfectly shows just that. She's the first one to jump into action when Sonic is arrested, beating even Tails to Prison Island. With Tails' assistance, she then sneaks into the prison, steals a security card and lets Sonic free. While she does fall into the background a bit until the last story once they get off Prison Island, she's still a fugitive on the run, a contributing member of Sonic's team and, if her appearance in 2P mode is to be taken as truth, her athletic skills have developed from slightly above average with her hammer to full on Badass Normal, with her able to play by Sonic's rules and decently keep pace with him. While she goes overboard in later games and becomes a hotheaded psycho, this game showed perfectly what a little bit of self confidence and determination can do for a girl whose only notable contributions prior to then were getting herself kidnapped.
Tails gets one in the game as well, for showing off that his engineering genius rivals, if not surpasses, Eggman's. How does Tails do this? The fake Chaos Emerald, of course. The Chaos Emeralds are ancient and powerful beyond belief, and Tails was able to create a replica, in spite of being less powerful, AND in spite of being it designed to destroy the Eclipse Cannon if put into the core. No one else has been able to accomplish this.
"Can you hear me, Sonic, Shadow? Everyone here is rooting for you! Good luck, and give him trouble!" - Rouge
When you realise that your reward is for getting all the emblems is Green Hill Zone in 3D, complete with the sound effects being replaced by the ones from the Genesis Era of Sonic. Before the location became mocked for being overused in most modern Sonic games, this was a welcome reward that made the harrowing task of 100% Completion all the more worth it.
Besides being very Heartwarming, having your Chao evolve into a Chaos Chao can be this, due to the hard work of raising it for so long. Also, it's immortal now, too.
The entire final battle just screams awesome. You've got two super-powered hedgehogs flying through space ramming themselves against a merged ultimate life-prototype/space station trying to defeat it before it crashes into the planet below and all while the game's main theme Live & Learn is blasting out, electric guitars and all. It's a worthy conclusion to one of the most fondly-remembered 3D Sonic games.
Seeing as how this is the first game to utilize a ranking system, getting an A Rank in this game is nothing short of this for the player. Especially if it's a particularly merciless stage like Crazy Gadget, Eternal Engine, Death Chamber, Mad Space, Final Rush, Final Chase, or Cannon's Core. And that's just for the first missions; getting it on Hard Mode should be awesome, and you should feel awesome.
Sonic: (whistles) That was cool! Tails:I did it, Sonic! Knuckles:Yeah, this is perfect! Shadow:Ultimate victory! Eggman:Now you know why I am the best! Rouge:How was that? Perfect? Like me~?
The final stage, Cannon's Core, is an entire level of awesome; as it's essentially five stages in one, with all the game's main playable charactersnote With the sole exception of Shadow, who joins in immediately afterward for the Biolizard boss battles. all working together to break into the core of the Space Colony ARK itself to prevent its fall to Earth. Needless to say, it's highly tense throughout, and a true test of your skills.
The entirety of Cosmic Wall is one for Eggman. Not only does it have low gravity so you can practically fly with your hover ability, but it also has by far the largest amount of robots in the game (if not the series) to blow your way through. There's even a couple sections where your walker's legs are restrained so you can focus on just blasting everything in sight. It's also available as a walker duel stage in two-player mode, making for an exciting battle as both players use the low gravity to try and outmaneuver each other.
. It perfectly displays the character relationship between Sonic and Shadow and sets a mood that if Shadow isn't stopped here, Eggman will take over the world — when played from the Heroes' side, anyway. The music just makes it all the more epic.Shadow: You never cease to surprise me, blue hedgehog. I thought that capsule you were in exploded in space.