Without a doubt it would be KG, his on-court theatrics are WAY over the top sometimes (the clapping in Calderon's face was totally bush-league, for example). I think the most frustrating thing about it is that it's so unnecessary. He is one of the best players in the league, he can make a statement just by his play, he doesn't need to drop more F bombs than The Big Lebowski and make a spectacle of himself every game.
I love him as a player, but some of the on-court antics wear on me a bit, and even as a Celtics fan, are tough to defend sometimes. I would imagine other team's fans despise him.