I like all of the Sonic game soundtracks I believe, but if I had to pick a few to name as my favorites, I would go with Sonic Adventure, Sonic Adventure 2, and Sonic 2 as my favorites. I would choose Sonic Adventure 2 as my overall favorite soundtrack as well. I say this due to how much I like each level’s song in each game, and how the theme songs for the games are as well.
If I had to pick an individual level’s song as my favorite, I would say Emerald Coast is my favorite Sonic song for a stage ever. For theme songs, Eggman’s theme from SA2B is awesome, as is a bunch of the other theme songs. I basically like all Sonic music, as most people do. I have heard some people say that no matter how bad a Sonic game may be, you can always count on the music being great. Other mentions for best individual level songs are City Escape, Final Chase, Green Hill Zone, Hot Crater Act 1, and Seaside Hill, to name a few. Too many songs to name; I would name all of them if I could. :P