I still play the non-combat OST from Far cry 4 if I need to relax.

Deus Ex: Human Revolution - Icarus Main Theme
If you won't feel anything while listening to that, you're basically dead inside. :hmmm:

MAFIA
 

A fabulous theme this; I have to let it play all the way through before I can begin the game:

https://www.youtube.com/watch?v=Hdy4U_DSCfU

2 games:

TES Morrowind. It's just right and pretty monumental which again makes it right for that game. Also, works well on a stereo if you don't mind short tracks on repeat like if you were say, a goldfish.

Ultima Underworld. Context sensitive soundtrack in a time when soundtracks were mostly beeps and boops. Added immensely to the gaming experience then as it still does. Few UW players can hear that tune today and not see themselves running around in dark caverns.

Rogue Spear

Medal of Honor (Playstation 1)

Another Medal of Honor (the main theme - wow!)

Donkey Kong Country 2

Rome: Total War


And my all-time favorite FPS - Wolfenstein 3D (Mac version): I played this as a kid until my eyes bled, this entire soundtrack will forever be etched into my brain.

I can't forget these:

Descent II Redbook Soundtrack - Cold Reality

https://www.youtube.com/watch?v=bIzA1LKW5mo

Silent Hunter 3 music - Battle 1

https://www.youtube.com/watch?v=eSenaycZqp8