Some pretty typical answers up in here... I wonder, do people think certain music is epic, simply because it is orchestrated?
Some of my favorite soundtracks are as follows, but not in any particular order:
Riven: The sequel to Myst
Myst 3: Exile
Super Castlevania IV - yes, i think it's better than Symphony of the Night. Really, all 'vania music should be on this list.
Panzer Dragoon Orta
Batman(NES) - this ain't no Danny Elfman PoS
Silent Hill series - notably, games 3 and 4.
Oddworld: Stranger's Wrath - ambience + chill groove + western = amazing
Sonic the Hedgehog 1 and 2 - I always did like Sonic music more than Mario, even though i preferred the Mario games.
Half-Life series ah, fuck it. Every later Valve game has got a exceptional soundtrack.
Call of Duty series (except WaW, I believe I became tonedeaf for a week after playing that game)
Company of Heroes
World in Conflict
Fallout 3 (Main theme stands for aprox 90 % of the election)
...And now I have probably listed every game in my gaming arsenal. Except Halo of course...