There are so many viable candidates for "worst music I've ever heard" - do we go with things I define as virtually indistinguishable from angry noise, music so annoying that I've resolved to hit anyone that attempts to play it in my proximity until such time as they stop, music that is objectively well executed while still being horribly jarring and unpleasant, or some other terrible quality?
Luckily for me most of the points I'd rant about have already been hit by other posters, freeing me up to post this:
From a strictly musical standpoint it's fairly benign, but that's why I'm posting Owl City's "Fireflies" in a worst music ever thread - you think it isn't really so bad, pretty wussy of course but not awful... and then he starts talking about giving a million bugs a million hugs.
And thus the reason I've posted it: That song is 100% pure sonic emasculation - if you can listen to it all the way through without turning it off in disgust, you are no longer a man[footnote]If you were never one to start with you just have shitty taste in music.[/footnote]. That's right, I'm asserting that if you ever listen to the entire song it will castrate you; claiming to be in full possession of a working set of testicles and liking "Fireflies" are fundamentally incompatible.