2D Sonic 3 & Knuckles (Sonic 3 + Sonic & Knuckles)
I really liked that each character could go through their own ways in the levels. Especially with Knuckles playing through almost all of the game differently. Having the second act in every level being different to first, in terms of appearance, was a nice touch. And the mini-boss feature was also pretty nice
3D Sonic Adventure 2
I liked that it had 3 different gameplay styles, and unlike the first adventure, this one didn't have any disappointing ones. Music wasn't bad, either, since the songs fit well with the levels, and were good enough to listen outside of the game too. The multiplayer was also pretty fun.