Subject line is self-explanatory so long as you take the word "worst" literally.
Please make them jokes you have heard.  Don't just make up the dumbest shit you can think of and post it.  No need to offer comment if you don't wish to.

I'll start:

"A naked blonde walks into a bar and orders a double entendre... so the bartender gives it to her."

"A traveller knocks on a farmhouse door and asks the farmer if he may spend the night.  The farmer says, "Alright, but you'll have share a bed with my son." And the traveller says, "Boy, am I in the wrong joke!"

"A newlywed is banging his bride in the honeymoon suite when he has the urge for a pack of Wrigley's. He figures it's late, no one's gonna see.  He hurries down he hall buck naked with his little soldier standing at full attention. Just as the aroused spouse retrieves the chewing gum from the dispenser six nuns walk around the corner. Newlywed panics and poses like a Greek statue fanning the gum sticks between his fingers and turning the other hand turned palm up. Mother Superior declares, "Oh, how adorable, a chewing gum vending machine!"  She gives the cank a turn, drops a quarter on the palm and takes a stick.  Sister Joan repeats the process, as do Sister Theresa, Sister Kathleen, and Sister Berandette.  Finally Sister Maria gives the crank a turn, but there are no more sticks of gum to be had, but the sister does not compalin.  On the way to the elavator, Mother Superior asks, "Sister Maria, did you get your gum?"  She replies, "No but I go some wonderful hand cream!"

Why they are bad:
The first lacks any wit at all.  The second relies on meta-joke with implied homophobia.  The third is an absurd and verbose set-up for a punchline about a man ejaculating on a nun. 

The only dirty joke that I can remember that might be bad is this one:

What do you call two lesbians in an ice cream shoppe?

Lickity Split.



Although it was kinda funny when I was in my teens, I'm trying to remember who told it to me.