Anti-Climax Boss: For many fans, the final boss, once you know the trick. Lure it to one side of the room, hit it before it recovers from the recoil of its stomp attack, and retreat to the other side of the room before it shoots its spiked arms at you. Rinse and repeat. No joke, this was the intended way to fight it; your biggest enemy is your own nerves, being the final boss and you have no rings. Similarly, the boss will be a cakewalk if you can get behind him and stay there. When behind the mech, it has no offensive moves except when standing still, leaving you free to hit its jetpack unopposed. He can't shoot his arms at Sonic, and he can't turn around. He can shoot bombs at you, but they drop in a set pattern, and only when he stands still.
Chemical Plant Zone is a common favorite, due to being one of the fastest levels in the Genesis games. It's in Sonic Generations for a reason! The other favorite is Casino Night Zone, which codified the Casino Park style level for both the series and for subsequent games. It was in the 3DS version of Generations instead.
Hidden Palace Zone in the remake.
Breather Boss: Mecha Sonic, the second-to-final boss. While it's still a tricky fight (the lack of rings, it's small hitbox and variety of attacks not helping your case) with just the right timing, he can be defeated in roughly 10 seconds. If anything, he's just a warm-up for the fight against the Death Egg Robot.
Breather Level: Sky Chase Zone, an easy and relaxed level with lovely, gentle music as you fly through the sky. Helps that it's after the horror that is the Metropolis Zone.
Sega's apparent decision to not release the remastered version on PC or consoles and keeping it on mobile devices has gotten a bit of hate, not helped by the additional content that will probably remain exclusive to the mobile versions.
Whether the Hidden Palace Zone in the remake should've used the Dummied Out track instead of Mystic Cave Zone's 2-player music. Some feel that Hidden Palace deserves its own track, while others consider "0x10" to be Soundtrack Dissonance.
Disappointing Last Level: The Death Egg. All the effort to reach it, and the final level is just two boss fights with no rings, meaning you are forced into Trial-and-Error Gameplay until you get their tricky patterns down, which depending on how many lives you have left can force you to start the entire game over again.
Demonic Spiders: All the badniks in Metropolis Zone are Goddamned Bats, but the Slicer (mantis) badniks stand out the most as Motherfucking Bats. Shellcrackers can be avoided by jumping over the large claw. While Asterons explode at the worst of times, they have spikes that fly out straight in a star pattern and thus can be avoided. Slicer's claws, however, are quite large, have the annoying ability to home in on Sonic, and are usually placed right in trial-and-error spots. Needless to say, they will get you at least once unless you are Super Sonic. Small wonder why many considered it the most hated badnik in the game, if not the most hated badnik in the Genesis-era Sonic the Hedgehog games, and would you like to guess who's back in the fourth game?
Ear Worm: The boss theme is very likely to get stuck in your head.
Franchise Original Sin: The almighty speed booster is introduced in Chemical Plant Zone. Not a problem here, but it caught Dimps' eye, leading to a common criticism of the Sonic Rush series and especially Sonic the Hedgehog 4 being the sheer overabundance of them throughout the game.
Sonic 2 would start the trend of every major title introducing at least one new character, in this case Tails. This initially wasn't a problem since Tails lacked the common writing problems and Gameplay Roulette that plagued future characters, but with each game that passes by with a new residential Base-Breaking Character or Scrappy, the novelty wore off and now any implication of adding a new character in the latest Sonic title can cause fans to lose it.
Game Breaker: If you plug in the second controller, Tails can be used to defeat Eggman's machines easily because of his invincibility.
Goddamned Bats: A few enemies stand out (such as the firefly Flasher enemies in Mystic Cave and the Aquis seahorses in Oil Ocean), but the biggest set of Goddamned Bats live in Metropolis Zone. Asterons, suicidal starfish bots that shoot spikes when they blow up, Slicers, praying mantises that throw their pincers like a boomerang, and Shellcrakers, crabs with huge fists that always seem to be positioned in awkward places. One of the main reasons why Metropolis is That One Level.
Sonic can't be hurt by the otherwise nasty Water Eggman boss in Chemical Plant Zone if he kneels, making it much easier to defeat. Interestingly, this doesn't work if you play as Tails. Also in Chemical Plant, there's one point where Sonic can go so fast that he can outrun the screen.
"Super Tails": Use the debug code, enable some means of getting all seven Chaos Emeralds (either code or genuinely) and transform, then generate a Teleport monitor and jump on it. His jump height is still too low to be really useful, and it goes away if he falls into a pit or gets crushed, though.
With objects that launch you (such as the see-saws in Hill Top Zone) if you spindash before you get launched, it will carry over and you'll shoot forward as soon as you hit the ground. This became an Ascended Glitch in Sonic Mania with the introduction of the Drop Dash.
Harsher in Hindsight: Oil Ocean Zone (a level in which the water is made entirely of oil dumped by refineries) becomes a lot more tragic when one accounts for real life oil spillages in seas and oceans, in particular the Deepwater Horizon accident.
It Was His Sled: Hidden Palace Zone in the remake is intended to be a surprise to players, but it's pretty much impossible to discuss this in any Sonic community without someone spoiling it.
Polished Port: The iOS/Android versions has Knuckles integrated in without needing Sonic & Knuckles, a Boss Rush mode, an extended vs. mode, an updated soundtrack, and both a finished version and a secret beta version of Hidden Palace. The game also has a couple new visual effects (most evidently the Special Stage, which has a very smooth framerate). This is due to being built on a new, custom engine designed to port classic Sonic games.
Porting Disaster: The PlayStation 3 version of the game has glitches happen far more often, such as removing hitboxes for some walls that, while you normally wouldn't run into, have springs.
When leaving a Special Stage, you lose all your rings, which can be frustrating for those who want to get all 7 emeralds early on, and also for when you exit a Special Stage from a pre-boss checkpoint, as boss arenas often have no rings. Averted when locked on to Sonic & Knuckles, however, as Knuckles's ring count when he touches a checkpoint is restored even after he loses a life. However, Knuckles' ring count is not retained in the iOS/Android port.
Having Tails tag along in the Special Stages can be frustrating if the CPU is controlling him, as you lose rings every time he gets hit. Even worse, sometimes he may get in front of Sonic if you hit a bomb, leading to him collecting all the rings and not you, and he'll most likely smack himself into a bomb and lose them all.
Signature Scene: This game's memorable ending was revisited in a few other Sonic games.
While not terribly difficult, Catcher Eggman at the end of Casino Night Zone is pretty tricky, and it can be hard to beat him with all your rings still intact by the end (tragic, considering how many rings one can end up with on this level).
Much the same applies to Flying Eggman, the boss of Metropolis Zone (except that, it being That One Level, you've probably lost most of your rings before you reach him).
The Wing Fortress Zone boss (Barrier Eggman), as you have to watch out for randomly moving spike platforms and a laser all at once, with limited space to move around. Dodging the laser is easy enough, but avoiding taking damage from the spike platforms isn't so easy, and you have to jump on the platforms to reach the laser and get a hit. Have fun if you lose all your rings.
Mecha Sonic, the penultimate boss fought aboard the Death Egg. Without rings. Mecha Sonic has a hitbox that's very small and you get no bounceback from spindashing into it, resulting in a lost life. If the fight goes on for long enough, it starts to shoot spikes during its spindash. Also, you'll have to fight it again if death occurs when fighting the Death Egg Robot.
While the Death Egg Robot is already difficult normally (with its spiked arms making it tricky to jump into it, the lack of rings against it, and it taking twelve hits as opposed to eight), when playing as Knuckles (by locking Sonic 2 onto Sonic & Knuckles), it becomes a nightmare thanks to his reduced jump height compared to Sonic and Tails, making it easier to jump into the spikes and impossible to sneak over them with a running jump.
Chemical Plant Zone. You know it's a wake-up call when Sonic Team themselves (or at least, the members who developed the Sonic Gems Collection) had trouble with this level. Aside from being a water level (without any air bubbles!), there's one segment where you must climb up a series of rotating stair blocks as water rises. You can potentially get crushed, or (heaven forbid) fall back down into the water and have to do it all over again.
Mystic Cave Zone, since literally almost every single object in the Zone is a trap of some sort, and the enemies are either hidden or small enough to overlook. Oh, and it has an inescapable spike pit in Act 2 (made worse if you happen to be Super with hundreds of rings). This was fixed in the remake, as the spike pit is replaced with a transition to a new level.
Some players consider Oil Ocean Zone to be one, although it's nowhere near as bad as Metropolis Zone, which comes right afterwards. The Aquis and Octus badniks that shoot at you are extremely hard to avoid, making the whole level a bit of a headache if you're trying to retain your rings. The Aquis can also fly at you from out of nowhere.
Metropolis Zone, mostly due to the enemies there being the toughest badniks in the game, and some are even placed poorly in the level (such as one of the Slicers being at the top of a yellow triangle spring wall you have to bounce up to, making it hard to avoid the boomerang blades and avoid running into the Slicer at the top of it). It is also the longest Zone in the entire game, bearing three Acts instead of the standard two.
Contested Sequel: In contrast to the near-universally beloved 16-bit entry, this one's a lot more divisive, with opinions ranging anywhere from it being a Sophomore Slump to being the best of the 8-bit entries.
Older Than They Think: The 8-bit version actually predates the Mega Drive/Genesis game by two months, and was Tails's first true appearance in the Sonic series.
Porting Disaster: The Game Gear port suffers from the camera being zoomed in on Sonic too much, but not having the levels compacted in order to compensate, leading to a lot of cheap deaths and leaps of faith required on behalf of the player.
Scrappy Mechanic: The infamous hang gliding sections from Sky High Zone, panned for the hang glider controls taking a great deal of time to get used to.
That One Boss: The Mecha Antlion (specifically on the Game Gear). It's considered tougher due to the smaller resolution mentioned above, making it harder to avoid the bouncing balls (you're also on a 90 degree slope that leads to the boss, and falling into the actual boss on the right of the screen is also easy to do). And it's the first boss in the game.* Making matters worse, some bombs fall low and others bounce high, whereas in the Master System version they all bounce low and are easily jumped over. Oh, by the way, after throwing the last bomb, Eggman does a fast ground-sweep charge along the ground that you barely have any time to jump over, plus it's possible for a bouncing bomb to still be coming down the slope when Eggman comes down. Every boss afterward is, for the most part, a pushover, though you don't get rings for any of them.
Aqua Lake Zone Act 2. For starters, it's completely underwater, and any Sonic fan would know what underwater means in this context: drowning. To make matters worse, it has patches of fake walls in rather nasty spots that you can accidentally fall in, and at least four big bubble dispensers you climb into to float upwards. The worst of these is near the end, which can fortunately be avoided by going through a fake wall, but if your speed is not maintained, you fall down and have to do that horrible "dodge the spikes + enemies hoping your bubble doesn't pop" section.* While you can jump right through the fake wall to get to the exit, you must take the pipe under it to be also to reach the zone's Chaos Emerald, which is required to reach the final level, and being unable to go back up the pipe after the Emerald will force you to deal with the final shaft with the spikes, spears and Badniks. Doesn't help that it's an enormous level compared to most of the others.
The third act of Green Hills, in comparison to the rest of the zone; it's a poster example of Platform Hell with hills, springs, and spikes, with the final approach to the boss being completely unforgiving in its timing. And no checkpoints means you have to do this again if the boss kills you.
Scrambled Egg Zone, thanks to the very elaborate and very confusing transportation pipe system, full of Trial-and-Error Gameplay as one wrong move can mean inescapable spike pits. There's only one pipe maze with spikes that you CAN climb out of near the end of Act 1, but Act 2 has several spike pipe puzzles, and the last one requires timing with no room for error, or it's a spike pit that is effectively a bottomless pit for Sonic.
What an Idiot: Just before Underground Zone's boss, Sonic is sent careening right for a pool of lava... and Eggman saves him from this most-definitely-fatal situation just so Sonic can fight the boss instead. Presumably Eggman had an The Only One Allowed to Defeat You mindset at the time.