Mine has to be when Ed from 'Ed, Edd & Eddy' randomly says 'Sush! My yeast is rising.' after Edd asks for his opinion on Ralphs traditions, even though he isn't baking anything!
I have lots of other favourites, but that one always makes me laugh for some reason.
One fav quote from ZIM is the old classic 'You LIE! You LIIIIIIIIIEEE!!!'.
Anyone else have a favourite quote?