One guy that stands out is former Olympic Bronze Medalist, 76' olympics, and former WBA Heavyweight belt holder "Big" John Tate.
those of you saying lennox and wlad need to read a history book. worst chin on a HW champ would be a guy who won the title and lost repeatedly, a shitty champion, think tommy burns, marvin hart, bob fitzsimmons, primo carnera, jersey joe walcott, bruce seldon, frank bruno, etc. guys like that