And although it's already been said I think Johnny Cash's 'Hurt' is probably the greatest cover of all time. Who and what he covered as well as how did it. Classic and contemporary. Genius!
Credence Clearwater Revival's version of the Gladys Knight / Marvyn Gaye hit 'I Heard It Through The Grapevine' is brilliant - much more raw and bluesy and John Fogarty's vocals give it more emotion.