I think that the music from Homeworld definitely deserves consideration. The combat themes help to highlight the different personalities of your enemies, and the thematic music for each music matches the deep-space atmosphere almost perfectly. The soundtrack was actually so popular that it was released as a separate CD in the "Game of the Year" edition.

Personally, I'd put their use of Samuel Barber's "Agnus Dei" (the choral version of his Adagio for Strings) at the top of any games-related music list, but they seem to be focusing on music especially composed for a game.