I could never hear it be called horrendous - it may not be as good as the original, but very few covers equal or surpass the originals. Some Beatles covers are technically brilliant, but all you'll ever hear is people spouting "Oh you can hear Lennon spinning in his grave".
Worst cover ever....
I really hated what Madonna did to American Pie - The song (to me) needs a really strong vocal, not some prissy weepy whiny type lead. Didn't do it for me one bit.
Best covers I hear you say?
All the stuff by Richard Cheese,
Goldfinger's 99 (red) balloons
Munkypunch's cover of the Tetris theme (google for them, you cn preview some of their stuff from a link on their site)
Reel Big Fish - Hungry like the wolf
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If at first you don't succeed, lower your standards.