Original, meaning, non-grand-theft-auto. To this day, a month doesn't pass where I don't give Final Fantasy 6 a listen, but my current (for quite some time) favorite scores for any game are Zone of the Enders 2, and Final Fantasy 12, the former even residing in my car's CD player because of it's frantic electrical/opratic/symphonic sound.
The ending music to PSO is tops. And I want to say that the expansion to PSO had two themes from the first Phantasy Star, which was nostalgia heaven. Not to derail, but the world needs another Phantasy Star. More than any other game, a single player sequel or prequel (since 4 brought the game to a satisfactory close) to that game would be blissful.
Too bad the PSP one isn't infrastructure, but hopefully it's mechanics are basyed on PSO, even if it's core is based on PSU.
Legend of Zelda: Ocarina of Time good music for each of the locales you visit, plus the songs you play to warp are pretty cool too. i have the CD where they were all done by an orchestra, definitely my fave
Just throwing out an answer that hasn't been said yet, but Chrono Cross. Some seriously fantastic game music there.
As far as a recent game, I think the soundtrack for Mass Effect could be up there. The score is just epic in that game.
Grand theft Auto 3. All original music, all listenable. I even have the entire Head Radio loop as an mp3. None of the other games listed have actual listenable music.
Except for this guy.. Scientist Wiki KJah is still my favorite station in that game though. Maybe not the game as a whole, but "Someday the Dream Will End" in FFX is one on my favorite video games songs.