My top 5 looks like this 1. George Chuvalo Never went down in his career, check who he fought http://boxrec.com/en/boxer/119 2. Oliver McCall Same as Chuvalo never went down http://boxrec.com/en/boxer/451 3. Foreman (comeback) He ate about 10,000 punches from Holyfield, Moorer and Morrison. 4. Ali http://boxrec.com/en/boxer/180 That's unbelievable in his later years 5. Randall Tex Cobb He just took beating after beating, Shavers and Holmes were the worst http://boxrec.com/en/boxer/2540
Mariusz Wach ( took a full 12 round beating from wlad throwing his hardest shots) In truth Wach has never been actually stopped ,the few times were waved off while he was still fighting back. Overall I would go with 90's Foreman .Fought some of the hardest punchers ever and was only nearly in trouble once when Holyfield hit him dozens of times at once. Very hard to hurt with a single punch just watch the Cooney and Morrison fights. . Hard to say after that because to many variables as some weren't hit as much or cleanly or hadn't fought as hard punchers. Certain proven top level chins in no order are , Ali, Mercer V. Klitschko , M. Tyson ,Holyfield ,McCall ,Valuev ,Holmes etc,,,,these types are very hard to hurt and can withstand many rounds of total power shots .
Ali because he took so much punishment off Frazier, most fighters couldn`t have taken what he took in Manilla and none of them could have taken the shots he took off Shavers, he also took bombs off Foreman.
Watch Lewis vs Briggs. Don't be fooled by the fact that Briggs was knocked out in 5th round. I don't know how anyone can withstand that kind of punishment. His chin is absolutely iron.
Chuvalo didn't do it for as long as McCall nor did he do it against men as big as McCall was pitted against.