Hmm, I would probably say Icewind Dale. Closely followed by one or two of the other infinity engine games, because the Baldurs Gate series and Planescape: Torment had some high points.
Silent Hill has some good music definitely.
Halo's music has always been one area where it rises up above much of the mediocrity of the games if you ask me, but occasionally it slips into cheesy territory. Not bad though.
Total Annihilation was also great.
Ehh, Im sure I can think up a few more, but it doesnt change the fact that Icewind Dale is my fav.
Edit: I did like Mass Effect's soundtrack, but its obviously a heavily inspired one. Still, I thought it was great all the same.