Here's a list of the bottom 100 cover songs:
http://retrocrush.buzznet.com/archive2007/badcovers/index.html
Personally, the worst cover I've ever heard is Milton Berle doing "Yellow Submarine." An appropriate song for such a giant talent, a fellow who was every inch
["foot," shurely?!] the gentleman.
Most risible would have to be any of Shatner's covers. Brent Spiner of TNG also tried his hand at this sort of thing, but he could carry a tune better than Shatner. he also managed to talk Patrick Stewart and some other TNG stars to sing with him.
Telly Savalas is another that's pretty bad/funny. Among other songs, he covered "You've Lost That Lovin' Feelin'" when he put out an album during the height of his "Kojak" stardom. The weird part is, he
spoke the lyrics, not sung them.