-Any Beatles song done by Joe Cocker
-Jimi' Hendrix doing Bob Dylan's "All Along the Watchtower"
-SRV doing Jimi Hendrix's "Little Wing"
-Patsy Cline doing Willie Nelson's "Crazy"
I agree on Joe Cocker for "A Little Help from my Friends" and Hendrix for "Watchtower". There are several brilliant Dylan songs that sounded better from someone else. "To make you feel my love" by Billy Joel, Garth Brooks, or even Trisha Yearwood (?!) comes to mind.
The WORST ever (shooting is too good for this crime) was Madonna butchering American Pie. My number 2 pick for worst ever is the South Park clobbering of "Come Sail Away". Even though it is supposed to be bad, it actually makes me mildly angry when I hear it.
Limp Biskit's cover of "Faith" is so bad I like it. It is really funny.
By the way (to whatever crazy person brought this up).. saying anything Michael Bolton ever did could compare to one note from Otis Redding is blasphemy. Redding had soul.. Bolton had fake/cheesy pop appeal to middle aged women (ala Barry Manilow).