Well, in anticipation of Sonic 4 Episode II coming out on PSN this month, I decided to start a Sonic the Hedgehog marathon. So far, I've got:
Sonic the Hedgehog (beaten)
Sonic the Hedgehog 2 (beaten, major improvement on the first Sonic)
Sonic Generations (beaten, right up there with Catherine for best game of 2011)
Sonic the Hedgehog CD (beaten, this was the only main 2D Sonic platform game I hadn't beaten until now)
Sonic the Hedgehog 3 (replaying, already beaten on Genesis)
Sonic & Knuckles (replaying, already beaten on Genesis)
Sonic 3 & Knuckles (replaying, already beaten on Genesis)
Sonic Advance (GBA, ongoing)
Sonic 4 Episode I (ongoing)
Sonic Spinball (already beaten on Genesis)
Sonic 3D Blast (ongoing, UGH I hate this game)
Sonic Colors (ongoing)
Sonic Adventure (already beaten on Dreamcast)
Sonic Adventure 2 (already beaten on Dreamcast)
Sonic Heroes (already beaten on Gamecube)
Sonic the Hedgehog Pocket Adventure (ongoing, hey, I need another reason to drag out my Neo-Geo Pocket Color)
Sonic CD was my recent finished game. A very unique experience, but it really is different from all the other Sonic titles. Japanese soundtrack was EXCELLENT, though.