Impossible to judge. Songs can be crap on all kinds of levels, either because they're lyrically stupid (My Humps), offensively bland (James Blunt), or simply a horrible smurfing noise (Crazy Frog). This is, of course, not counting the songs that aren't terrible in themselves, but so horribly overplayed and overrated that you can't help but tire of them (I Predict a Riot looks to be heading in this direction). While you might think you've found the worst song in the world, there's always something else out there that's appaling on a whole other level. So it would be foolish to pick out one song as being terrible above all others.

...Oh wait, my mistake. Avril Lavigne's cover of Chop Suey is uncontestably the worst crime ever committed against human hearing. I pray for the day when all copies of that song, and Miss Lavigne herself, are strapped to a rocket and fired directly into a black hole.