For some reason Halo: Reach always stuck with me, particularly when it really kicks in at 1:00. Possibly because it sounds like it should be in a Dragon Age game or something, which admittedly was always what I wished I was playing when I was compelled to play this with friends!
https://youtu.be/Hr6dlbhLKdk?t=59I agree with Ori, for sure. I've only Bean Dived it, and yet that title music and the music of the first level alone have made it shoot right to the top of my recovery list.
The Skyrim theme edges it out for me over the Oblivion one, just because the latter is a little more generic. Plus it reminds me of the incredible launch trailer that had me so damn excited.
https://www.youtube.com/watch?v=AgByF6KT-M8Vigil would probably go straight to the top for me, though, along with most of the trilogy's score.
As for more sombre themes, I have a real soft spot for Assassins Creed: Revelations. It's so much more haunting and ghostly than the game itself turned out to be, which is a shame!
https://www.youtube.com/watch?v=SaYupMWjn68