This song has the honor of being one of the saddest and most beautiful songs I know. It was a hard decision to cut it to one, because Lea Salonga is so amazing.
Honorable Mentions:
Also from Les Mis, On My Own & I Dreamed A Dream (Both by Lea Salonga)
Nemo, by Nightwish. I miss you Tarja.
Amaranth, also by Nightwish. You're great too, Annette.
Beethoven's Ninth. (Actually, probably the most beautiful song, but there aren't any youtube videos that do it justice.)
Music of the Night, from Phantom of the Opera.
And lots more. But that's enough for now.