It seems almost trite to mention it, simply because of its ubiquity...but it stands almost alone in music, for me.
Samuel Barber - Adagio for Strings
Also, Barber's choral 'agnus dei' arrangement has a special power. The version I know best is the one from Homeworld
. If you've played the game you'll remember it as the music on the third mission when they come back to find Kharak destroyed while they were away. That is one of the most affecting moments in any game, for me. Maybe it's something to do with home.