guns n roses "live and let die".
ha ha, just kidding, they both suck infinitely, so it's really a tie.
seconded on the devo, and voivod's cover of astronomy, but i have to say that the voivod Nile Song is so terrible, i can't imagine how bad the original would have to be...
i'd say #1 most massive improvement for me would be Cannibal Corpse covering Sabbath's "Zero The Hero". just hearing him say "love-a-duckin'" is in itself priceless.
Cannibal Corpse, Hammer Smashed Face EP
i would also say Polvo's version of Mexican Radio, but the original was pretty damn good, so... maybe a tie there.
Anthrax version of Sabbath Bloody Sabbath was great in that the gutars were way heavier, but replacing Ozzy with Joey makes it harder to say it's overall an improvement. but musically, i think so.
RUN-DMC "walk this way" is a bit of a slam dunk.
what about Zepplin's "Gallows Pole"?
can i make a joke now about Elastica covering Wire?