Idk why Magellan is on here, he didn't even come close to winning.
Almost certainly Crocodile though, his entire ability involves him absorbing water but for some reason it only works on his right hand and doesn't work when it's a fist coated in water. But apparently it does work when luffy is eating him. And when he gets coated in liquid he can't absorb it from himself with any hand. And I have no idea how haki-less luffy manage to punch through sand blades that could split the desert in two.
He mummified luffy but luffy got saved because some water he tried to spit at crocodile earlier fell back down on him, yes this is an actual thing that happened...
Then robin pulled an antidote to crocodile's poison out of her ass and luffy got a nakama power up from vivi and won
Wouldn't have even been so offensive if Oda didn't have the nerve to then give him a 100m bounty after all that