Personally, I enjoyed Sonic Adventure more so than probably any Mario game.
Sonic Adventure was the absolute SHIZNIT. In fact, I might pull out my Dreamcast soon just to play that, and Shenmue of course.
There was also a Sonic Adventure 2 on the DC but I never had that one. Might look for a cheap one on eBay. Also debating getting Sonic Unleashed for the Wii as I've heard good things.