Whole soundtrack from Dune. Especially the version for Sound Blaster, without wave synth, but CD "Spice Opera" is great too!

I suppose we are limited to only one song.
So my top choice is <span class="bold">Myst 3 Exile - Main Theme</span>.

I think the answer really depends on whether the music has to be composed specifically for a game or whether it is simply used in a game. Big difference really. If the former then I'd have to point to the main theme to Outlaws by Clint Bajakian.

If you open it up to licensed music composed for some other reason (say for a movie, rock songs, etc.) then my answer would be very different.

Final Fantasy Crystal Chronicles has a couple of nice songs, my pick is the title theme: Kaze No Ne

I don't speak Japanese, so I'm just judging it on a musical basis. I really like Yae's voice and the harmonies and arrangement are lovely. The English release of the game 'dubbed' these songs and the American singer uses far too much vibrato and doesn't pull off the harmonies nearly as well, I'd rather listen to beautiful singing that I don't understand.

As a bonus, here's the credits theme from the Japanese version: Starry Moonlit Night Can't have a song without vocals...

one winged angel