I love the Sonic 2D games, save for Spinball and 3D Blast. 3D Blast is tolerable, so is Spinball. Don't particularly like them though. Never played Chaotix, but I want to.

I really liked Sonic Adventure as well, one of my first games for the Dreamcast. I liked everthing except for the parts with Big the Cat. Everything else was pretty decent. I haven't played anything after that though out of the 3D era.