I really liked portions of the DRagon Age soundtrack, but they're disqualified for not including the Ostagar theme on the album they released (and, indeed, making the track they title Ostagar "Orzammar"...grade 1, people!). By dint of there being little to no original music in any of them, I am also discounting the Star Wars games.
Deus Ex, Planescape: Torment, Starcraft II, Warcraft III (in fact, the Warcraft series in general)...it's hard to pick just one. If I had to, I would have to say Crusader: No Regret. Not only a great selection of tunes, but a command to switch music at will while playing the game!