I'm going to say Sonic Adventure 2. Sonic Unleashed would be my second choice. I'll put Sonic 4 just in case you love it.

Sonic 2 and 3 are my favorites.

You forgot Sonic 2006.

Many are going to say Sonic 3, I suspect, but I prefer Sonic 2. Probably for nostalgia reasons, and because Sonic 3 had some very maze-like levels that I hated. Especially that darned pyramid stage.

Sonic 3 and Knuckles. What else could you ask for. Two sonic games in one.

Sonic and Knuckles is by far the best. With slick controls, stunning graphics and excellent music this game is simply sublime...

Where's Sonic Heroes? That's my favorite.

Sonic 3 locked-on to Sonic & Knuckles. Hyper Sonic rules.

Everybody in the history of forever knows that Sonic CD is and will always be the best Sonic game evar.

sa2b is my favorite. only really solid 3-d sonic game. the only problem: sonic's last level is riddled with shortcuts.

You forgot the Advance and DS games.
Also Sonic Riders and Sonic Rivals.

I like SA2, and in my opinion, it was the last good 3-D Sonic game.

Can't really beat Sonic 3 & Knuckles, in my opinion.

Mainly because of Super Tails' "Stand Still To Win" ability in boss battles.

I just LOVED Sonic 2. Sonic and Knuckles was good too.

Sonic Adventure 2 is not only the best Sonic game but one of the best video games ever. The series was all downhill from there.

Oddly enough, I never did get much into the original 2D games, even though I have Mega Collection for the Gamecube.

Sonic Adventure 2 is certainly the best 3D Sonic game, and the series did go downhill from there, but I wouldn't call it the best. That spot is reserved for Sonic CD. :3 You really should try to get into the original 2D games, though.

Everyone likes Sonic CD, but everyone knows Sonic Adventure is better
o3o.

Sonic 2 is awesome!