I really like Knuckles Chaotix. It's a little on the easy side but it looks great and it's just fun to play
I have to say, I'm a huge fan of Knuckles Chaotix. I think the issue with it is that it's not a straight up Sonic game, it has some unique gameplay elements that make it different. Some people like that, some people don't (especially those who "gotta go fast"). To me, Sonic CD had pushed the 2D Sonic formula as far as it could go, so mixing it up with the chained second character (and resulting piuzzle elements) was a nice change of pace.