For me, it would have to either be "Dante's Prayer" by Loreena McKennitt or "Left Behind" by VNV Nation. "Dante's Prayer" because the music is just gorgeous and the way she sings it is beautiful. Not to mention the choir parts leading in and out of the song add so much poignancy to it. And she can somehow put so much emotion into three little words: "Please remember me...."
"Left Behind" is a wonderful song as well. The music isn't much, but somehow I don't think it's supposed to be. The words are so full of meaning and emotion it's unbelievable. I have a theory that everyone can relate to this song in some way. And not only is it incredibly depressing yet gorgeous in its descriptions, the end offers a little glimmer of hope which just adds to the overall impact.
Overall, just two great songs.