Sonic 1 has awful pacing problems. You can blast through Green Hill Zone in about 2 minutes ish. And it's great! Fun, high speed platforming, nice sense of flow, etc. Then that all comes to a grinding halt with Marble Zone. It's slow paced and requires precision platforming, which given the way Sonic handles (a lot of momentum, hard to influence direction once a jump has been made) can be very difficult. It is overall the antithesis of Green Hill. Spring Yard Zone is a step in the right direction, but suffers from its own problems. You can go fast, but as a result you may get pinball'd all over the place, which can get frustrating/hold you up, or be blindsided by awkwardly placed enemies. Labyrinth Zone is everything awful about Marble Zone times 10, thanks to slow underwater movement. It isn't until Starlight Zone where the game regains its sense of speed, and even then the level design can be irritating with tons of bottomless pits and occasionally confusing level layout. The game feels like one set of really great levels designed around the game's physics and controls, and then a ton of basic platforming stuff. Sonic 1 has a lot of problems that keep it from being a great experience.
Sonic 2 and 3&K, on the other hand, are great games that fix a lot of 1's problems. Sonic 2 manages to nail down pacing, with every game in the zone having a solid sense of speed. Sonic 3&K make the levels feel more expansive without breaking up the flow. It has these massive levels with tons of different paths that weave in-and-out of each other, allowing you to switch from path to path easily without bringing the game to a crawl. 2 and 3&K feel like they are design around you going fast, and have levels that are designed in such a way that speed is a reward for fast reflexes and using the environment around you to your advantage.
So yeah, Sonic 1 doesn't really hold up the best. But I don't think you have much to fear with how well 2 and 3&K hold up.
