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
my fav is Sonic 2 closely followed by sonic adventure 2 battle. i liked sonic 2 for reasons i think everyone here can understand and i liked adventure 2 because you could play as all the characters and of course the super awesome, amazing chao garden. i probably played the chao garden more then the game.
My favorite Sonic game has to be Sonic the Hedgehog 2 for the Sega Genesis, even though I have to say I liked Sonic Adventure series and the rest of rhe Sonic the Hedghog series for the Genesis, but overall Sonc the Hedgehog 2 is my all time favorite.