The Xenoblade series has such great OSTs in general. If I have to pick just one track it's probably Engage the Enemy
Gaming
I regularly listen to the Subnautica OST while working.
The only game soundtrack I ever recall make a point of listening to is from Faster Than Light, it's some really good ambient music.
I'm currently hooked on the Bomberman 64 soundtrack again. That intro cutscene music and the world select still please my eardrums!
silent hill and no more heroes are both franchises with amazing soundtracks.
Darksiders II has a phenomenal OST. I still listen to that regularly.
That being said the Burnout 3 music was great but obviously not an OST
Favorite instrumental soundtrack themes are from the early Silent Hill games. The theme from SH1 and the Theme of Laura from SH2 are classics.
Favorite lyrical songs used within the game for plot: Alan Wake has characters in it who are 70s rock gods called Old Gods of Asgard, and the creators got a group named Poets of the Fall to actually write the two songs that were featured from the game and they are actually really great songs. Poet and the Muse is a plot point and Children of the Elder God is played during a really good sequence.
Diablo IV has a killer soundtrack.