Hmm... I'll echo the sentiment that the last truly 'great' sonic game was Sonic 3+knuckles

The last good one was definitely Sonic Unleashed on PS3/360. The day stages were a lot of fun and captured the whole Sonic idea quite well. The night stages were a farce and kind of ruined the game in a lot of ways though. I'd buy a new Sonic game if it was simply the Sonic Unleashed engine with only normal 'day' stages. Yes I would. Trying to compete in time trials in those and such was incredibly fun and to really cut the seconds off took a lot of practice.

The Sonic RPG by Bioware deserves an honorable mention for at least being a good attempt at a game that wasnt a total cashin.

Also, Sonic Colors on the Wii is actually pretty fun. Not a spectacular game, but the ideas are compelling enough to be worth playing.