Luffy. Whenever he tries to be poetic before a fight he can sometimes get out of control and end up making no sense out of what he is saying thus confusing is enemy, unwillingly of course which results in Luffy getting a more frustrated due to the fact his enemy usually says something daft in response then Luffy kicks arse. Nuff Said
I have to go with Usopp, I was asked this a while back by some friends and it will always be Usopp, his cowardliness makes me laugh but when his friends get hurt and he can't bear it the emotion breaks out and he can kick some serious butt... and yet again like I said him being a coward is hilarious! Oh and Sanjii he is funny too.