not really sure how to judge chin since most flush clean punches will put a man down. however the most "un-knockoutable" man in history is definitely ali imo.
Very correct Bill B. A freak knockdown to Marty Marshall, And a 'legitimate' one-punch knock-out loss to Leotis Martin in December 1969, when he was 42 years-old, and 'totally exhausted'. My 'sleeper choice' Adolfo Viruet. Only suffered one 'flash knockdown' in his entire career. Fought * Monroe Brooks * Pete Ranzany * Roberto Duran * Bruce Curry * Ray Leonard * Donald Curry All when they were in their 'prime'.
Marshall broke Liston's jaw in the 4th round, but I don't believe he knocked Liston down in that fight. I may be wrong.