I agree with David Tua as a good choice. Years ago Randall "Tex" Cobb was known for his chin although his defensive skills were so poor that even the iron-chinned Tex got stopped a few times.

Here's something I've noticed for awhile: iron-chinned fighters rarely clinch. This is because they're willing to get into a slugfest and trade heavy shots on the inside. That's my kind of fighting so I'm quite partial to that type of fighter.

George Foreman had an iron chin, but like Tex Cobb his defensive skills were not good enough, at least in the 1st part of his career. During his comeback he was never knocked down in spite of facing a number of big punchers such as Morrison, Holyfield, Cooney and Alex Stewart. I think George deserves serious consideration for having the greatest chin ever in spite of his loss to Ali (exhaustion and carelessness) and his knockdowns suffered at the hands of Ron Lyle (total defensive meltdown) and Jimmy Young (exhaustion again).

In the old days, Jake La Motta was known for his iron chin. Pound for pound La Motta's chin might have been the best ever.