I don't feel that novelty/christmas songs, like many that have been already mentioned, can be judged aesthetically. They're inconsequential. The artists don't release these tracks in the manner of music as an artform. It's music as a saleable commodity. It's when poor music gets released with complete sincerity, and receives high acclaim, that questions really have to be asked. That, in my opinion, is what qualifies a worst song.
Still, there are too many poor 'good' songs to pick an all-time worst, but I think I can manage to nominate one from the last few months.
Perhaps it's down to sheep frequency of play, but my nomination is for James Blunt's You're Beautiful. What an absolutely dire excuse for music. I don't necessarily dislike the acoustic-pop sound this takes from, but how this has become an anthem for people I just can't fathom. It's just wet without anything to justify the sentiment. Give me something to work with Mr Blunt! "I saw your face in a crowded place / And I don't know what to do / 'cause I'll never be with you." Wow, now that's pathos!
I'm with you Phalanx, what's "bewtifah" supposed to be, and what on Earth is an "ayn-gah"? Perhaps its success is because it's so easy to sing when you're off your rocks on the night-bus. No need to have the ability to pronounce words in a fashion that resembles their original form. Or maybe it's because he's "one of our boys in the field".
I rarely have such a vitriolic reation to music; I try to embrace as much as I can and find some merit, but I failed miserably with this. Makes me feel sick, deep inside, when I hear it... oh, wait, maybe that was the idea.
Damn, he's a clever one.