The GnR one, the Ashley Simpson and that dutch one can't count. In the case of GnR it's not bad really, it's not great, but far from terrible. The Ashley Simpson one can't count simply because it's all backing track and her bad vocals alone doesn't qualify it. The opera one can't count cause it's just one, really REALLY bad note. And the dutch one was supposed to be bad.
So far it's a tie between Nirvana cover, that weird hardcore song, and The Final Countdown.