When I say worst, I mean worst. A joke with no redeeming qualities.
Here’s my contender, from the book “1000 Knock-Knock Jokes for Kids”:
– Knock Knock.
– Who’s there?
– Ann who?
– An apple fell on my head.
There’s something beautiful about this one. It’s the clerihew of jokes. Zero cleverness. It lacks any sense of inevitability, in that any sentence whatsoever could work here, as long as it begins with the word “An.”