Sorry, but I'm old school. I grew up playing the original Sonic, Sonic 2, then 3. It's a tie inbetween all 3 for me, depending on my mood, what levles I really dont want to deal with at the time and the bonus games to get the choas emeralds (need to be in the right mood for each game, but I find all 3 enjoyable). I found Sonic & Knuckles and 3D Blast to be the point of no return. Sonic was not as good in 3D, in my opinion - just way to much of the game play is out of your control.
SEGA all-star racing (sure not really a sonic game but...) was great, as well as Sonic 4