2D: Sonic the Hedgehog 3. So much
Sonic CD and Sonic the Hedgehog 2, too. I enjoy their awesome music just as much as the gameplay, which is why I can't stop playing them. XD
3D: Love Sonic Adventure 2: Battle
That's my most favorite game of the series, mostly for the story, the music and the Chao Garden. I also enjoy Sonic Riders, Sonic Colors, and Sonic Unleashed. And I know Sonic Unleashed gets a lot of flak because of the Werehog factor, but that's actually one thing I like about it. I love platformers, so that gave me a good fix for that.
I REALLY want to play Sonic Generations though. I heard that game was
fantastic Spin-off: Sonic R is one of my favorites ^^ Tails Adventure is good, too, though I haven't played it in quite a long time.
As for one I don't like: Sonic and the Secret Rings. As much as I love Sonic, the controls for that game just pissed me off so hard :U But that's about it. I haven't really played Sonic '06, the one everyone hates with a passion, so I have no opinion on it