It's apparently more sample than cover, but Karl Wolf's version of Africa by Toto is a tough one for me essentially right off the bat. Can't even get to the first verse.
Not the worst, but:
While I do like some Aerosmith songs, I can't say their cover of Come Together by The Beatles belongs anywhere near that list.
Regarding U2's cover of CCR's Fortunate Son - "It ain't them."