In no particular order:
Quake had a bunch of awesome tracks (NIN, so that's a given XD) but this one was probably my favourite:
Others already touched on some of my favourite iD songs but for me, at least, The Imp's Song and Sign of Evil are the best.
And I'm not done with just those either!
Red Faction: Accused
Red Faction: Umbra Pogo
Descent 2: Gunner Down
Descent 2: "Turnabout Bore" (why don't we get songs like this anymore?)
Trackmania: Rally Race
Trackmania Sunrise: Fill Your Pages
And finally, although it's from an old mod instead of an old game
Would want to add some tracks from Sonic Unleashed too but it doesn't feel old enough to me. I came to it late.