I'd have to say my two favorite video game soundtracks have come from Mirror's Edge and Dead Space (1 and 2). Dead Space has the classic horror mix into a more dark presence. It feels like some older horror flicks when a monster pops up, and it's done so well. Jason Graves is the man when it comes to horror music, definitely makes things intense. Mirror's Edge just had a melodic atmosphere amongst it that really brings out the best in its world. The soundtrack for ME couldn't have been better for it.
Man Mirror's Edge is a beautiful game. I can't wait for more info on the second one. <3