Starcraft Terran music (Terran 1's second half is probably the best track). Also, Starcraft 2 Terran music (other races are good but not quite as epic in both games).
DX music is great too.
Half-Life 2's soundtrack was amazing at what it did, but it was very situational (tuned to the setting, that is), and thus it gets an honourable mention. I actually have the DX and Starcraft soundtracks and I listen to them on occasion; I would not do the same with HL2.