Too much a toss up.. Sonic 2 was more fun with that Debug cheat and super sonic mode! Woot!! I still rem that code too heheh..
I remember going to Germany and making the kids at the game store AWE at me.. Seems Germans dont use cheat codes eh? (Took me awhile to rem it that night too lol)
But Sonic 3 was nice due to the fact Tails was more useful in there! Plus it was fun playing the game with just him. =)
I only wish Sonic 2 and Knuckles could be ahem.. easily found and not just 3 and knuckles.