I'm replaying the Genesis Sonic games on Steam. My order is:
Sonic 3&K > Sonic 2 > Sonic 1
Leaving out CD since I haven't replayed it yet.
Sonic 1 seems to be more of a straight up platformer with less emphasis on speed. That's okay, but I find the later games to be a more fun mix. Marble Zone, Labyrinth Zone, and the final zone are pretty annoying since there doesn't seem to be much you can do to speed them up. It's just a boring slog. Sonic 2 refined the graphics, music, and controls quite a bit and the levels are more interesting. The bonus stages suck, though. I can't see what's coming around all these twists and turns, the low framerate makes it hard to watch and Tails makes me see red.
Now let me gush a little about why I love 3&K:
- The graphics, music, and controls are improved even more.
- The two acts are more differentiated and offer more unique gimmicks. I especially love that act 2 now has its own variation on the stage theme.
- The split second shield is great since it allows a skill-based way to take out spiked enemies without just waiting around for them to become vulnerable.
- The elemental shields add some variety to Sonic's moves, which is great because...
- The game is long as hell.
- I love the slightly more cinematic approach they took. Sonic 2 felt like a sequence of stages until Sky Chase Zone, whereas 3&K feels like an enormous adventure with an actual story and characters, yet it's told without dialogue and without interrupting the gameplay much at all.
- Speaking of, some of the setpieces are awesome. The wall of death in Hydrocity Zone, the snowboarding, the Lava Reef (mini?)boss, Hidden Palace Zone, the fight with Knuckles followed by Robotnik stealing the Master Emerald, and the unforgettable final confrontation in Doomsday Zone.