Batman and the Joker are the best characters (yes yes I'm obvious).
Something that always makes me laugh in the game, I'm assuming someone else will have noticed this, is when you kick an enemy to the ground and perform a ground takedown, Batman sometimes lands the wrong way round, so his erm... well, balls will be in the guys mouth pretty much. He'll then either punch him in the nuts or the arse and "Ground Pound" will pop up at the side of the screen.
Whenever he punches them in the arse that cracks me up every time.