Bored, have a top five.
5. A big ol' tie between Sonic 2, Pocket Adventure (surprisingly good for such an obscure game), S3K (a true classic but a fucking marathon), and Sonic Advance
4. Sonic CD 2011 (AKA the first game I didn't suck at)
3. The Sonic 4 games, particularly E2 but they're all fun
2. Sonic Lost World (so addicted to this game, gets a bad rep for no real reason and the Deadly Six are the greatest new characters in over a decade. Go ahead, shoot me)
1. Sonic Generations (A really fun game even though I don't talk about it too much. I'd play it way more if I didn't suck at competing in it, but seriously, this game is PACKED with content and it's pretty much everything Unleashed and Colors did but better)
As for the other games...well, I enjoy most of the spinoffs (except Black Knight, Spinball, Labyrinth and sorta Secret Rings) though I haven't played them all. Most of the 2D sidescrollers are also really good, I even like Sonic 1 if I'm in the right mood. Colors is okay. I don't have enough experience with Unleashed to judge, but it doesn't interest me much. Haven't played Rivals or the Rush series yet and I don't think I need to.
3D Blast, SADX (except Tails story), SA2B, the majority of Heroes, Shadow and the majority of 06 can all go suck it. Tails spinoffs aren't too amazing either.