The Elder Scrolls V: Skyrim, and The Elder Scrolls IV: Oblivion have really nice soundtracks. Both made by Jeremy Soule, I believe, whom you can maybe look up for more game OSTs and stuff. My favorite one is from Oblivion, called Auriel's Ascension. 'Tis pretty neat. ^_^
There's Morrowind, too, out of the TES series. In fact, if you already have those... What game OSTs so you already have?