>>20
>Sonic 3 & Knuckles
>Sonic Adventure 2
Why would my favorite be one that isn't the best? That said, CD comes awfully close to Sonic 3 & Knuckles, especially Taxman's version. The sheer amount of content in these two is so much more than in the rest of the 2D games. In 3&K it's because of more levels and more characters, in CD it's because each level has a lot of exploration potential and actual reasons to explore (Robotnik's machines, Metal Sonic Holograms, plus of course time travel opportunities), and the time trial mode with actual unlockables helps a ton.
Sonic Adventure 2 likewise has more content than most of the 3D games. The story mode is only a tiny fraction of the game, it doesn't really begin until you finish that and get to the Extra Missions. The unlockables are spaced just far enough apart to keep you motivated to keep playing, and the final unlockable for getting 100% on the game is probably the best in the series.
Other 3D games, like Unleashed, have an awful lot of content too, but I like Adventure Sonic's gameplay better than Modern Sonic's. I feel like it's a more accurate translation of Classic Sonic to 3D, so Adventure 2 gets my vote.
For the record, Unleashed is best/favorite Modern Sonic game. People bitch about the Werehog but it accomplishes what it sets out to do well, and there's just so much to do in the game. Generations is too short and easy, even to get 100%.