I think the original is the best. The level design just seems.......how can I say it...tighter than in other games. There are none of those 3D pipes or long running sections,which have hardly add anything to gameplay, and the game is the most challenging in the series. And I also prefer the simpler controls, the 'spinning charge' in the other games did not really add much IMO.
Sonic CD is another great one, it tries to do everything bigger and better than the original, but the level design is a bit sloppy.
