Not counting spinoffs and other stuff with Sonic:
Universally agreed to be good:
Adventure 1 and 2 (NOT the DX versions)
Generations (no need to play the original now that Sonic X Shadow is out)
Frontiers
Debatably good:
Heroes
Unleashed
Colors
Not good:
Forces
06
Boom
Shadow
Black Knight mystical rings whatever this stuff was
I think thats all the major ones