I do not have just one funniest overall episode,
but there are parts of episodes that I found hilarious.
My favorite is probably when the Doctor
was poisoned in The Unicorn and the Wasp.
When Donna and the Doctor
meet again in Partners in Crime.
In The Doctor Dances
when the Doctor and Jack
are comparing their sonic "stuff."
Eleven, Amy, & Rory
have had some funny moments as well.
I could keep picking funny moments forever!
But I will stop here.