Game soundtracks that are good for musical/compositional reasons:
Hitman 2 (particularly the End Boss track, not composed by Jesper Kyd, but by a hungarian composer, whose name has slipped my mind)
Many Bill Brown soundtracks, like RTCW, Undying and Trespasser etc, though they are admittedly mostly pastiche pieces written to sound like other composers,
Some tracks from the Final Fantasy games are pretty cool and even sound quite original.
Silent Hill series, particularly the first and second. They're not works of art by any means, but they're very effective, chilling and mostly original sounding.
The tracks from the Medal of Honour games which DON'T sound too much like John Williams are pretty good. Otherwise they're just cheesy.
Soundtracks that I like for more nostalgic reasons, where the music isn't actually that great:
Pilotwings 64 (especially Birdman!)
Tomb Raider 1 and 2 (the music here is actually pretty good but it's the atmosphere that does it for me)
Resident Evil 1 and 2
And many other games soundtracks of many years ago.