If we're going with original soundtracks then I would have to say the Halo Series, specifically 1 &2.
Now best soundtrack of actual record label songs goes to ATV Offroad Fury 2, it had the best compilation of music that went perfectly with the gameplay.
GTA Vice City and San Andreas have fantastic soundtracks, but they're not originally composed for the game or anything. There's a lot of really good original radio bits and ads written for the game though.
Out of the games I've played; The elder scrolls and halo series. My personal favourite though is the soundtrack for the game Total Annihilation; composed by Jeremy Soule who also did TES. Major kudos to anyone else who's played it.
Ok, I love Bethesda, but their music wasn't anything special. Sure, those long nights you spent playing Fallout 3 and Oblivion sort of implanted a mark in your heart, and i do get excited hearing them, but these aren't ood enough to be favorites, but it is your opinion, so i'll shut up now...O.O
Ah come on the boss battle music was epic. It was the best reason to go back and beat the game. Just to hear that awesome music. And to think it all is modified version of the Teddy Bear picnic song