And that makes 100% completion today.
Scrap Brain isn't as bad as Labyrinth Zone but it's close. Pro tip: Go really fucking slow. I know, it sucks, but that is how you beat it. Every single trap feels like it's asking you to rush past it so you miss signposting and get zapped or murdered, and there are blocks that you have to wait to appear. Enemy design isn't nearly as bad as Labyrinth Zone, though, just traps are worse. That being said, having to play through it so slow to function is distressing, if not the antithesis to what the game seemingly wants?
The final boss? No rings. I thankfully got through it unscathed, finding the electric balls easy to react to, and positioning myself so I always had an out. Never got hit once, although asking for the player to never get hit is still asking too much. Made the boss kind of a joke for me, though. If you're impatient, you're dead.

The last three achievements on RetroAchievements were invented by people who felt like abusing the player, at least in my book. Completing Marble Zone in less than 90 seconds is a neat challenge, and actually not that hard to do. The underachiever and 200 rings achievements? Ridiculous. The underachiever asks you to wait 5 minutes in every stage of Green Hill, and have 0 rings when you get there, AND have hit no enemies. Green Hill is already a slightly unfair stage, but now it's completely jacked up, and waiting is painful without speed on emulation. But the 200 rings might be the worst one simply because most stages are in the 210-230 range if they're even close, which means backtracking through each path of a level. I walked out of a stage with only 198 because some were impossible to get in one go, and said fuck it. Time for cheats. I'm not sorry.

24 of 24 achievements.

Star Light might be the only decent zone besides Green Hill, or at least Act 1 and Act 3 are. Act 1 is a lot more straightforward in how to be speedy, and the platforming isn't nearly as bad as previous slow sections. That being said, Act 2 has the new bomb enemy in spades in dickish corners, another level that just screams "WHY".

18 of 24 achievements

18 of 24 achievements

Spring Yard is not a good stage, I don't know why people defend any part of it. Filled with speed traps, annoying sections, constant pace breakers, and just way too many spikes! There is no going fast here, at least not without being wildly out of control, and very likely going to hit something that will hurt. At least you're feeling pain while listening to good music?

Labyrinth Zone might just be the worst experience I've had in a platformer that wasn't, like, broken or bad from the beginning. If you can get to the end of any Act with 50 rings, I applaud you. Terribly design around platforms that will constantly drop you and sometimes unavoidable damage, along with INSANELY dickish enemy placement (and enemies themselves!). Also the special stages? Kill me. The last two are brutal. I hate this shit so much.

15 of 24 achievements

Had to replay Green Hill for some achievements, specifically collecting rings in the first seconds of a level. For the record, Green Hill's song is good, but I've been so overexposed I can no longer say it's amazing, if it ever was.
Marble Zone isn't exactly as bad as I was told it would be. Don't get me wrong, it's still bad, but it's not on the level of throwing a controller. It's the first time Sonic 1 confronts the player with having to learn the platforming if they hadn't done so in Green Hill, which can be unfair if you were just trying to go fast and keep momentum (which the physics system pushes) instead of stopping or exploring. Otherwise, rings telegraph jumps and the enemies aren't terribly placed. The problem is how SLOW it is, having to wait for blocks, spike traps, cycles, etc. And that means you're dealing with the not-great movement even more! There are safety barriers in the form of lava instead of spikes, which thankfully don't insta-kill you like spikes (which is... who thought that was ok), and usually falling has another platform on the top, but it's disheartening often to lose progress from getting hit that makes you track through the same shit you just went through, even if you didn't necessarily lose a life.
Good song though.

Had to replay Green Hill again for some more missed achievements, specifically 100000 points and to get an extra life I missed. Also got 2 emeralds from Marble, and can I just say special stages suck? The first one wasn't too bad, but the second one? The music isn't even good either, it's actually kinda annoying.

11 of 24 achievements

Going to go between short spurts and long ones for this. Green Hill is nice and clearly the best level here, but it still has annoying enemy placement designed to make you stop and reactions that are just too close to perfection required. Getting the best pathway shows how much the physics system and Sonic's movement kinda sucks for slow platforming, being insanely slippery and easy to fall off blocks, best seen in Act 2 and 3 of Green Hill. 4 of 24 retro achievements so far. Managed to net 4 unique checkpoints.