This is one of those things where you can't go wrong with any option. Always going back and forth between Aokiji and Kizaru, but these days it's Kizaru, Kuzan doesn't hit quite the same since he left the navy. His navy run >> vigilante/pirate run (so far atleast). Akainu is an elite character all the way through as the terminator of One Piece, and he's grown on me, but I still mess with Kizaru and Aokiji more since they aren't always seething 24/7 (Marineford Akainu was different though, dude is the best shittalker in One Piece).
-Aramaki: has been great too. Love how cutthroat he is, straight to business and no sugarcoating.
-Fujitora: I respect as a character, he's just not a favourite of mine (he has a nasty df though).
-Sengoku has no sauce whatsoever, so he has no shot at competing with the rest for me.