In no particular order; The Halo series (CE, 2, 3, Reach) has some fantastic pieces I'd recommend without a doubt, Elder Scrolls: Oblivion is best decribed as harmonious, tranquil and peaceful, lastly Mass Effect 2 which has pride of place on my MP3 player.
Last night an Escapist posted two pieces from Victoria 2 that I enjoyed so much I bought the soundtrack on Amazon immediately.