Imogen Heaps cover of "Hallelujah" is positively heart-wrenching. I want it played at my funeral.
I don't know if they're better, but Yael Naim and Static Lullaby both did different (and very good) covers of Britney Spear's "Toxic".
Lily Allen did "Seven Nation Army", that was pretty good.
She also did "Womanizer" by Britney. THAT was amazing. It sounded so classy.