I thought Sonic Adventure was a very good game. Apart from the Shadow/Sonic sections, the sequel was padded out with so much fluff that I lost interest and haven't bought a 3D Sonic since. I played the first level of Heroes and that 3D one on PS3/360, and I knew they weren't for me.
Sonic Advance 1 & 2 - especially 2 - were enjoyable (haven't played much of the 3rd) , as was Sonic Rush, and I'm looking forward to Sonic 4. There's a definite pattern here.