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 Level 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.
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. Additionally, it has the most 1UPs and Rings out of any zone in the entire game, with the very first act alone having a whopping four 1UPs and well over 200 rings. This makes (re)stocking on lives insanely easy note run thru most of the level collecting the goods, die towards the end, and repeat until you're satisfied on lives. And you'll need every last one of them for the rest of the game, too, starting with Green Hills's thirdact.
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 mobile remakes 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.
Cut Song: Track 10 in the 16-bit game's sound test is an unused and hauntingly good song that was originally meant for Hidden Palace Zone, and ended up unused when the Zone itself was Dummied Out. Even when Hidden Palace Zone returned in the 2013 remake, the song remained unused (the version uses the 2 Player Mystic Cave Zone theme, which is what it used in leaked prototypes).
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 themes for the Genesis and the Game Gear editions are both very likely to get stuck in your head.
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.
The Genesis version introduced the almighty speed booster in Chemical Plant Zone. Not a problem here, but it caught Dimps' eye...
Game Breaker: If you plug in the second controller, Tails can be used to defeat Robotnik'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 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.
It Was His Sled: Hidden Palace Zone in the smartphone ports is intended to be a surprise to players, but it's pretty much impossible to discuss those ports in any Sonic community without someone spoiling them for you.
The bad ending of the Sega 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.
Polished Port: The iOS version has Knuckles integrated in without needing Sonic and Knuckles, a Boss Rush mode, an extended VS mode, an updated soundtrack, and both a finished version of Hidden Palace and a secret version of the 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 Game Gear port of the Sega 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. Averted when locked on to Sonic & Knuckles, however: Knuckles's ring count when he touches a checkpoint is restored even after he loses a life.
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.
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. note Making matters worse, some boulders fall low and others bounce high, whereas in the Master System version they all bounced low and were easily jumped over. Every boss afterward is, comparatively, a pushover, though you don't get rings for any of them.
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.
The third act of Green Hills, in comparison to the rest of the zone, is also a contender; it's a poster example of Platform Hell with hills, springs, and spikes, with the final approach to the boss being completely unforgiving in it's timing. And no checkpoints means you have to do this again if the boss kills you.
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. 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.
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.
What an Idiot: In the 8-bit version, just before the first boss, Sonic is sent careening right for a pool of lava...and Robotnik saves him from this most-definitely-fatal situation just so Sonic can fight the boss instead. Presumably Robotnik had a "The Only One Allowed to Defeat You" mindset at the time.