I love the the music in the old PC game Fury of the Furries and some of the town themes in Heroes of Might and Magic 3.
The Tristram tune in the first Diablo was also great. I haven't played the game in five years or more, but still remember the music.
But, like MasterOLightning said, I don't listen to game OSTs voluntarily.
Bad music won't break a good game for me(the Burnout series spring to mind, as well as Devil May Cry 3, the battle theme song, that is), since you can turn it off at will, and good music won't save a half-baked game. I guess, what I'm trying to say is that I view in-game music more like a supplement to the experience rather than something that stands on its own.