Definitely Hallelujah! I really like the Rufus Wainwright (I think?) version though.

There's a song on the latest Kathleen Edwards cd, I think it's called Run. It is beautiful and haunting.

For classical music I am much more into big showy bombastic pieces (think, Stravinsky's Firebird) but Adagio for Strings is an amazingly beautiful piece of music.

I'm pretty sure our high school concert band played Pictures of an Exhibition, I'll have to dig it up and listen to it!