In no particular order (added as they come to mind), these are game soundtracks that rock my socks off. I try to stay away from most of the already-established franchises that are so famous for their music, or otherwise those games would dominate this list.
While the licensed songs are plentiful and amazing, there's also a good ambient original score here that doesn't get nearly as much recognition.
Composer: Peter McConnell
Favorite track: Rescue from the Sea of Tears
This soundtrack has a distinct digital character to it and contains great remixes of classic Metroid remixes along with original pieces.
Composers: Kenji Yamamoto, Kouichi Kyuuma
Favorite track: Phendrana Drifts Main Theme
A strange one, this. It is in fact a mix of old tracks from the original Half-Life 2 along with new ones specifically for the sequel. However, I prefer this one as it contains the best of both worlds. It gives off an oppressive mood.
Composer: Kelly Bailey
Favorite track: CP Violation
I'd say this is a very epic cinematic soundtrack, fitting for lots of explosions.
Composers: Michael Giacchino, Chris Tilton
Favorite track: Explosion Scherzo
Really good and varied fantasy soundtrack.
Composer: Nobue Uematsu
Favorite track: An Ancient Fortress
