It would be hard for me to pick a "worst song ever", but I will say that I really detest the new song by the Killers ("When You Were Young"). I wasn't a HUGE fan of their old stuff, but it was still enjoyable to listen to. Now it seems like they're trying to be taken really seriously, and it's backfired. The lyrics are pretentious quasi-poetic bullhockey, and Brandon Flowers sounds like a friggin' goat for most of the song before going into some really ridiculous falsetto at the end. Also, he's said in an interview that the new record was going to sound "more American", and of course, on this song he's singing in a ridiculously exaggerated "American" accent with overemphasized R's and ay's and whatnot. (For the record, they ARE American.)
Some of the songs mentioned are guilty pleasures ("Macarena", "Who Let the Dogs Out", "Surfing Bird", etc). I do agree with the person who said that "Brown Eyed Girl" is horribly overplayed, though I wouldn't call it the worst song ever. And as I said in another thread, Weird Al owns.