Marvin Hagler .... Arguably one of the best chins of all time.. revenged his two early career losses (questionable decisions) with TKOs .. and beat SRL ( my opinion ) , Duran and Hearns. I feel he gets a bit under appreciated, due to the SRL decision.
in my opinion, toughness is not simply being able to be a human punching bag, but also to be able to dish out viscous damage, and to be as unbeatable as possible. This is why Hagler tops Lamotta .
Salvador Sanchez might not have been as physically imposing, but his stamina was the best. Im sorry my respects for Hagler, Marciano, Pacquiao, Mayweather, RJ ,etc Id here some of these guys even train on holidays, but Sanchez never got tired or even gasped for air in a sport as grueling as boxing. Wolf in sheeps clothing for me its Sanchez and wasnt even in his prime yet.
The toughest prizefighter of them all was LW champion Battling Nelson. He took some legendary beatings and always came back for more. The circumstances in those days was totally different from what we see today. We are talking about 45 rounds and fights to the finish. Little to no rules enforced. 4 ounce horsehair gloves.
Hagler is up there. Roberto Duran anyone? James Toney is also one of the toughest. A non-HW boxer who got in there with big punchers like Rahman or Peter and held his own. Never been stopped or close to been stopped in 80+ pro fights.