The question is the best for their weight classes, but I feel extra recognition should be given to those who must have had the absolute best chins-which has to be a HW, or theoretically it could be a rehydrated CW who is about the same weight, or comes in the ring ~ 10 lbs. heavier than Chuvalo...Just because even the LHWs will not hit as hard. I am going with Oliver McCall. I welcome any challenges, my case is: 1) The very rare HW who was never down. Also never down as an amateur or in sparring. No fight even stopped due to injury or cuts. 2) Unlike someone like Marion Wilson, did not have a style that thwarted attacks or tied you up a lot...Meaning the actual punch resilience is tested. "The Atomic Bull" sums it up. 3) 59-14-0 with 436 rounds & his last fight was at 54! (Just turned 55). Has had about a *35* year career. 4) While a few may have fought greater average bangers, he faced some bombers, & modern HW sized guys. 5) 300 + rounds as a sparring partner vs. the unsparing Tyson, who used to go through them like water. 6) Bruno hit like a truck, 38 of his 40 wins by KOs. he & Lewis could not dent him. Even though LL teed off on him when completely open & defenseless & having some sort of a nervous breakdown. 7) Given his problems with drugs, mental illness & run ins with the law, that he never hit the canvas is especially impressive. This all adds up to me as.... The Greatest & most Proven Natural Chin in Boxing History. It would take an Atomic Bomb to even Drop the Atomic Bull.