Mmm, thing about Kenpachi is that he's not going to kill you though. He may maim you beyond belief to get his enjoyment out of a fight, but when you're lying on the ground like Ikkaku it becomes pointless for him to continue. I don't really fear him so much because of that.
I wouldn't want to fight Mayuri. That sick freak would have you in an agonizing science project until death if he won--if he didn't kill you outright (egh, by poison after being paralyzed no less

).