I love the music to Canabalt and Tiny wings and even did a budget remix of the latter's theme as I loved it so much.
Gears has the best music I have heard in all games so far. There are others of course - Ravenmark, Contrejour to mention a few are great too, but Gears triumphs them all.
I also like the music in World Of Goo, it really fits the environment. Tiny Wings music is also great and I quite like Russian Dancing Men, it's pretty funny (probably will buy the music IAP, thats supposed to be coming out)
Where do u go to find game music in iTunes? I can't ever find anything... I thought alot of games had they're music in the store? I like the music in where's my water and Contre Jour etc....