Seriously, no one has yet to mention Gary Jules version of Mad World?
Or Jeff Buckley's version of Hellaluja?
What is wrong with you people?
Jeff's Hallelujah sounded awful to me. One minute I'd be turning up the volume so I could understand his incoherent whispers, then I'd be blasted away when he started screaming. I hate to speak ill of a dead man, but that is not my idea of a good cover.
I Wanna Love You // The Maine
Not exactly classical, but good nonetheless. ^^
EDIT: Oh yeah.
Mr. Sandman // Sara Bareilles
It's not a well-known recording, but I like it much better than the original.