The two grossest I recall were:

There once was a girl from Azores
Who's snatch was all covered with sores.
The dogs in the street
Would eat the green meat
That hung in festoons from her drawers.

There was a man named MacTavish
Who was known as a sexual savage
While fucking his mother
He beat off his brother
And ate out his sister's miscarriage.

Many more were renowned for cleverness ~

There was a lad named Crockett
tried to screw a light socket
Some son-of-a-bitch
turned on the switch
and Crockett took off like a rocket.