Arlong took a gatling, a bullet, a whip, and a bell without flinching. Then had his teeth shattered once, ripped out four times (Once by Luffy to free his arm, second time by Luffy to get teeth for himself, and twice by Arlong himself), got speared to the chest, impacted with concrete, Impacted with a building (Both during Shark on Darts), Had his nose shattered and then reset (The resetting is even more painful), and then finally had a five story building collapse on top of him.
Yeah, I think by now all of us already got the point that Fist Fist Ace thinks Arlong had the most punishment handed to him so I don't see why you need to keep emphasizing it.
Anyway for me it has to be Arlong, Crocodile and Rob Lucci. For every other opponent Luffy has faced, it took a decent hit to send them flying. But those three I've mentioned are clearly a level above and always refuse to go down. Kind of fitting that when they were each defeated, Luffy was issued a new bounty.