I've never found Joker's lines particularly funny in the comics, though I've always had trouble deriving humor from print. I don't know; I'm a very auditory person. It's all in the delivery for me.
So, needless to say, the funniest Joker bits for me have been in live action and animation.
Nicholson goes without saying; but let me think of some of his best moments -
-"Honey, you'll never believe what happened to me today!" The little head tilt, with the martini in hand, and the little hat - drop dead hysterical.
-"He stole my balloons!" - I would just keep repeating this scene over and over again, heh.
- (to gargoyle) "WHAT ARE YOU LAUGHING AT?!" - It's partially the look on the gargoyle's face, but that part slays me.
Of course, Hamill had some classics -
- The Joker fish commercial - "Tasty, tempting, and of course - naturally low in colesterol."
And yes, Romero sends me into giggle fits from time to time. It's fun seeing him be completely silly. "Well pip-pip to you sir!"