This'll have to be mine, but it's a tough one between the Mantis shrimp, Porcupine Puffer, Sailfin Tang, Mandarinfish and this, my Porcelain crab.
Not your typical contender, but I love this guy. At 2 minutes in, you see him swing a good right hook at the boisterous Clarkii clown. (who's since grown to twice that size and picked the crab up, carried it away from the anemone, and dropped it. The Crab now hosts various next bests.
(Crank up the quality) :)