I thought Dragon Age: Origins OST was incredible, Inon Zur is clearly a master at what he does. Especially with the main theme, which I found was beautiful and very haunting.
The Dragon Age 2 OST was also amazing, but didn't quite capture the soul the way the original did.
Auriel's Ascension in the Oblivion OST, I also thought was really great.
And not to forget Fallout 3 OST, because there is nothing better than romping through a wasteland violently beheading mutants to the joyous and uplifting melodies of Billie Holiday.