For bad covers I think I ought to mention that version of Under the Bridge by All Saints (I used to wind up one of my friends who was a huge fan of RHCP by telling him they were planning to cover Never Ever....)
While I'm here though, I'd like to mention Laibach's version of War as a brilliant cover. It takes the classic anti-war anthem and imbues it with (post-)modern cynicism, all the while managing to stay an anti-war anthem.