SADX, for sure.
Adventure fields, great old-age tropical levels, glitches that allow fun stuff, spindash gave you great accel that should be in every game, and of course the cinamatic action that Sonic Team worked so hard for.
Other opions:
SA2:B
Although the music/story were far superior, and the spindash and stuff were still there, it missed some parts of the Sonic-feeling. Sonic never really got in his running animation by just running, and too much levels were exclusive for a certain person, while I rather want to play as Sonic only. I only like the game for certain levels that are really entertaining. Who ever wants to replay the Knuckles mines, or those Rouge "get the emerald in 5 minutes" missions? More of Sonic, please.
Sonic heroes:
The idea was fine, and so was the max speed. Levels were okay to me, but this game lacked many things. The story was very confusing, how they tried to force players to slow down by switching to power/fly type sucked, and it was kindof a game-on-rail. Most of my time playing this game, I just tried to dump those irritating team-members, or I tried to run trough levels without using power/fly. The camera was kinda hard on you, as well.
Shadow the hedgehog:
Alright, this game missed a LOT of things. First of all, control! I found shadow kinda uncontrolable. Then you have the bad camera and guns. Now that sucks. The levels....let me be fair, the design of them sucked, and I'm really not waiting on hero/dark missions. Although I am glad that you can play them all with Shadow himself, instead of Rouge/Knuckles/Tails/Big/etc. The story can be confusing at times. Finally, the superfight! It took me so long to get it, and all I got was a Sonic heroes styled shooting match. I've lend the game to a friend to I never took it back.
Sonic Riders:
Needs more levels. Thats all I can say. Its so annoying that I have to do those 6 levels over and over again. Those near-copys of levels were just no fun to play. The story? Well, lets say.....why doesn't Sonic act quickly, when he needs to chase the "bandits". He can run at supersonic speeds, but instead he forces himself to use boards that can only go at 160 miles a hour.
Sonic genesis games:
I never really enjoyed any of them. All of em had a major mistake, making me not like that game. Sonic CD? Too slow, even in stardust speedway. Sonic 1? Wow, now I didn't like that. Sonic 3&Knux? Very weird movement and too extreme made level textures. Sonic 2? I found this the most enjoyable Sonic game. It had speed, good looks, and was simple. I didn't really like the Super Sonic system, though. Kinda buggy....