This looks like the perfect thread to shamelessly self-promote myself. I really like the Lost Odyssey main theme and I made a vid where I took the events of Halo Reach and put them to the song.
I do like the works of Jesper Kyd on Hitman: BM and the entire AC series as well as the music in the Splinter Cell Trilogy (The Crystal Method, Amon Tobin, Jesper Kyd) and, of course, the Halo games. Surprisingly I like the ODST soundtrack the most next to the original. TES games also have great ambient music that I listen to while doing other work. I'll let others post these songs as I just want to advertise my own vid for now.
EDIT: How could I forget the Zelda soundtracks? That 25th Anniversary Ochestral CD is beyond words.