This isn't the worst cover ever, it's the worst THING ever:
Someone mentioned Disturbed's cover of "Land of Confusion" and I have to second that; first time I heard it I was like, "congratulations, Disturbed, you managed to make Phil Collins sound like a total badass by comparison"
A few more that get under my skin:
Joss Stone - "Don't Fear the Reaper" (zzzzzzzzz)
The Ataris - "The Boys of Summer" (again, trying to make a pop song edgy making the original seem 1000 times edgier)
311 - "Love Song" (yes, THE CURE's "Love Song")
HIM - "Wicked Game" - not the worst cover ever, but easily the most unneccessary
Does anybody remember the Metallica tribute concert that ran on MTV some years back? That was epic: Avril Lavigne covering "Fuel", Korn covering "One", Snoop Dogg covering "Sad But True" .... there are no words.