I have just finished playing Halo 2 & 3 and boy does that music get you in the mood to (in an epic way) kill some aliens.
I believe that the soundtrack to a game is one of the most important components (even more important than the graphics). Metal Gear Solid and Final Fantasy 8, to just name two, having theme songs that I have been humming ever since I heard them
What are, in your opinion, some good soundtracks in games?
I love video game music. Consequently it's impossible for me to pick a single soundtrack as the best. Martin O'Donnell does the Halo franchise justice, the Okami soundtrack is bloody beautiful, Yoko Shimomura and Utada Hikaru in Kingdom Hearts are bloody brilliant, Nobuo Uematsu is one of my favorite living composers and has done a lot of Square games (I own every single Final Fantasy soundtrack, if that says something)... Metal Gear Solid definitely has a great soundtrack... And of course, Legend of Zelda and Mario have some classic pieces of music. Shadow of the Colossus was a work of art, made that much more amazing by the soundtrack.
While we're talking about video game music, I highly recommend this site to anyone as devoted to video games and video game music as I am: www.ocremix.org. It's a site devoted to "the appreciation, preservation, and interpretation of video game music."