Johnny Cash's cover of Hurt (NIN)
1,000 Homo Dj's cover of Supernaut (Black Sabbath - only Sabbath cover i've heard better than the original, most sabbath covers are fucking terrible - honourable mention tho to White Zombie's cover of Children of the Grave, quality cover)
and one im sure has been mentioned many times - Hendrix's cover of All Along the Watchtower (Dylan)
ah and one i came across very recently
https://www.youtube.com/watch?v=fzxag7U3Snk Daniela Andrade cover of Crazy (Gnarls Barkley - i didn't even like the original but the cover is cracking)