I'm a musician (several genres, from southern rock to blues to classical) and I really appreciate it when a game has a good soundtrack. I'd have to say the best soundtrack is LucasArt's Outlaws hands down. The game is pretty much just another fps (it's got some unique stuff to it, though, like reloading), but the music makes it awesome. It gives it feel that wouldn't be there without it. If you like games with good soundtracks, check e-bay for Outlaws. It's worth listening to the CD by itself easily. Although nowhere near Outlaws, Total Annihilation had a pretty cool soundtrack. Nice and melancholy; added a dark feel to the game. 7th Guest had a really cool tango song called "Skeletons in the Closet" by The Fat Man, too. Parts of Descent 2's soundtrack wasn't bad, either.
--Alan