These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Anti-Climax Boss: For many people, the final boss of the Genesis version. 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.
The boss is much harder with Knuckles because of Knuckles shorter jump you need to be more precise with your jumps.
Best Zone Ever: 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.
With the Taxman/Stealth mobile ports come Hidden Palace Zone.
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 Robotnik's giant 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.
From the Master System version, Green Hills Zone. Seriously, due to its setting and easiness compared to the rest of the game, it's like they cut what was supposed to be the first level and pasted it in the middle of the game.
Broken Base: 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.
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?
Franchise Original Sin: The Game Gear version's bad ending implies that Tails was murdered. Several years later, one of the biggest criticisms of the games are that the Darker and Edgier tone is incredibly jarring and sometimes frustrating in a series about Funny Animals fighting an evil fat man.
Game Breaker: Tails can be used to defeat Robotnik's machines easily if you plug in the second controller because of Tails' 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 the 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 Chemical Plant boss 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. Demonstrated in this speed-run of Act 1.
Magnum Opus: A similar contender to S3&K for best Sonic game in the series. It improved on the original game, making levels much larger, introducing Super Sonic, and some of the key mechanics of the series.
The bad ending of the Master System/ Game Gear version implies that Tails is dead. Trauma Ensues. You have to collect all of the Chaos Emeralds in the game to free Tails; if you don't, Sonic never faces Eggman. Instead, he dashes alone until nightfall, only stopping to look up into the sky and see an image of Tails. The implication is either that Tails was forever held captive or that he was killed. The music doesn't help any either.
Thought the infamous drowning music only plays when you're about to drown or hit the 10-minute time limit? Well, in 2-player mode, when one player finishes, the other player has 60 seconds to finish the stage or it's TIME OVER for them. So the time limit ticks down to 11 seconds and...yeah, have fun playing 2-player with someone significantly slower or faster than you now.
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 of the Master System game 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.
The PlayStation 3 version of the game has glitches happen far more often, removes hitboxes for some walls that, while you normally wouldn't run into, have springs, and can soft lock if you try to go super at the end of a stage.
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.
The loss of rings when entering a special stage in the Genesis version, which can be frustrating for those who want to get all 7 emeralds early on.
Signature Scene: This game's memorable ending was revisited in a few other Sonic games.
From the Genesis version: Silver Sonic and Metal Robotnik, the penultimate and final bosses respectively, which are fought aboard the Death Egg. Without rings. Silver Sonic has a hitbox that's very small and you get no bounceback from spindashing into it, resulting in a lost life. Towards the end of the fight, it starts to shoot spikes during its spindash. Metal Robotnik protects its weak point with its spiked fists, leaving only a small opportunity to successfully hit the weak point. Touching the fists or legs results in death, and you'll have to fight Silver Sonic once again if death occurs. The latter (along with other bosses) is made even harder when playing as Knuckles (made playable with locking Sonic 2 onto the Sonic And Knuckles cartridge) due to his lower jump height.
The Underground Zone boss in the Game Gear version is considered tougher due to the smaller resolution mentioned above, making it harder to avoid the bouncing balls. And that's the first boss in the game. Every boss afterward is, comparatively, a pushover.
While not terribly difficult, Robotnik 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 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, 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.
For the 8-bit version, Aqua Lake Zone Act 2 could very well contend for the title. 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. Doesn't help that it's an enormous level compared to most of the others.
Scrambled Egg Zone (8-Bit), 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.
Metropolis Zone (GEN), mostly due to the enemies there being the toughest non-boss enemies 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.
Also, Chemical Plant Zone (GEN). Not as hard as Metropolis, but definitely a wake-up call. Sonic Team themselves (at least, the members who developed the Sonic Gems Collection) had trouble with this level.
Some consider Mystic Cave Zone That One Level as well, 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).
Fixed in the Android version, in which the spike pit is replaced with a transition to a new level.
Some players consider Oil Ocean Zone to be one too, although it's nowhere near as bad as Metropolis Zone, which comes right afterwards. The sea horse and octopus 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 sea horses can also fly at you from out of nowhere.