The first Sonic game I ever truly played in full (despite having some part-time exposure to Sonic on the Genesis) was Sonic Adventure 2 Battle on the GCN. So, there's some fondness for it even if I know there are some major flaws with it.
But, recently, I went on a quest to really understand Sonic. I went and played Sonic 1-3, CD, and Advance 1-3. As well as Generations.
I like 2 the best of the original trilogy, followed by 3 (which mostly I dislike for the graphics and music being too different from the first two games). From the Advance trilogy I like the first game. The two followups had too much of a reliance on speed boosters and springs. I was really really impressed with Generations though. That was a great game.