Yes, Ladies and Gentlemen, I have found it!
Now bear in mind, this isn't necessarily the worst song ever recorded, bar none. It is impossible for anyone to state that, even myself--at least to a reasonable certainty. I mean, 20,000 songs are released every year. Who can listen to all of those?
But there is one song I feel quite comfortable in endorsing as the worst song ever to hit the charts. It was a 1971 hit by Lou Stallman and Bobby Susser, performed under the name Think: "(Things Get a Little Easier) Once You Understand." (Yes, yes, so it only hit Number 23 on the charts. But that's Top 40! It counts!) The link in the song title will take you to a page that contains a Real Audio file. Give this one a listen; it's horrible, sure, but you have to hear it to understand. And things get a little easier once you understand. (Thanks to Culture Raven for the link.)
Did I say horrible? I meant a colossal disaster. A train wreck. The Manos, the Hands of Fate of pop singles.
And yet...it's a puzzle. This flaming sack of dog poopey is a message-song (well, more like a jingle with some spoken melodramas over top) aimed at bridging the generation gap...but who is it that things will get a little easier for, once they understand? The parents? Or the kids? And what do they now understand? There's actually considerable debate about this; I've read commentary from people whose junior-high-school guidance counselor made them listen to it, and heard stories of parents whose teenage sons solemnly gave them the record and said "Just listen." But neither side can say for sure that their opposition is the one who needs to understand in order for things to get easier.
If it's the kid who now understands...well, for one thing, what the fuck does the kid understand now? He's DEAD!!!! If he can't manage a reflexive action, like, say, DRAWING A BREATH, he probably can't much wrap his head around "obey your parents and don't do drugs." Assuming that's what the late Robert Cook is supposed to learn, of course; alternative lessons apparently include "Mom doesn't trust you because you're a big fat liar," "You're wasting your life away with foolish things (but my bridge club and ladies' groups and my daytime programs, that's different)," "If you can't figure that out for yourself you're stupid," and "There's a little bit more to life than joining a group and playing guitar." Whichever lesson is the correct one, apparently the only way to learn it is The Hard Way: death by drug overdose. So much for the girl's mother who says "Someday you'll thank me."