Twyce said:
I found nothing captured a games essence more than this track from Silent Hill 2. It fits the tone and emotions of the game perfectly.
I second that. SH2 was one of the best gaming experiences of my life, and the soundtrack played a large part in that. Those melancholic tunes, combined with the incredibly unsettling "music" during battles and such, made for a very memorable experience. That old trilogy kinda reminded me about Twin Peaks, just way more twisted version of it, that is.
Other standout soundtracks:
Metroid Prime
Mario series in general (seriously, even non-gamers can probably hum three or four songs out of the mario series. Then you know as a musical director you did something right).
Final Fantasy VI
Secret of Mana (might be the best soundtrack in an RPG ever)
Zelda IV: A Link to the Past
WipEout (some might prefer the fully licensed 2097 soundtrack, but to me the first one will always be the best. Groundbreaking at the time, and the in-house work of Cold Storage was stunning. I remember other producers and DJ's bought the game just to be able to hear his work. Now that's how you make videogames cool!)