I love Sonic CD, it has this dark undertone that you don't find in most of the Sonic games. And it was developed by the same team that did Sonic 1, which is still probably my favorite Sonic game. Sonic 2 and Sonic 3 are still great, but to be honest, I've always liked Sonic 1 and Sonic CD better. Starting with 2, they concentrated too much on speed, and not enough on interesting platforming (often interesting platforming slows things down).