So many to list, I am not sure where to start. IMO, a great cover keeps the flavor and is immediately recognizable as the original, but definitely feels like something new and identifiable as the artist doing it.
Probably my all time favorite cover is The Damned's cover of Love's Alone Again Or:
Deftones did a whole cover album with some great tracks. I liked this cover of The Cure's If Only Tonight We Could Sleep. I like the Cure. I like Deftones, so it it is a great mashup.
Godsmack doing Beatles - nice heavy twist
Great cover of Dream On by Blacktop Mojo. Again, great heavy twist
White Stripes doing Dolly Parton's Jolene - I was impressed by this one. Keeps a bit of the original feel, but really they make it their own too