One more...
Shaquille O'Neil. Ever hear him talk? His singing's even worse.
I remember around the time he made an album, there were a couple of basketball guys making albums. The dinger from Digital Underground had a great quote in Sports Illustrated - "These guys deserve a recording contract as much as I deserve an NBA contract."
Then again, none of those guys or the ones in my previous post would have gotten a recording contract had it not been for their celebrity status, so it's not really fair to the original intent of the thread I guess.
I wasn't truly old enough to appreciate Yoko's heyday, so I can't truly dog her out. She's got to be the best of the worst though.
My pick - the "No-talent ass-clown" Micheal Bolton.