Manny Pacquiao spring to mind. No one can deny he's an ATG, he's also suffered ko losses and being down a few times.
What I will say about that Fergy is Pacquiao was very young in those KO losses, he was still a boy/young man growing into a man. His punch resistance improved alot when he put on weight and fully developed into a man, and the KO loss to Marquez would of knocked an elephant.
While i have the upmost respect for the fans of Thomas Hitman Hearns, he did suffer knockout losses in big fights. He was stopped by Sugar Ray Leonard on Sept 16 1981 in round 14 of their title unification bout after building a lead. He was later stopped by World Middleweight Champion Marvelous Marvin Hagler in round 3, on April 15 1985 after a furious three rounds by both combatants. Then on June 6 1988 Hearns was stopped by Iran Barkley in round 3, after winning the first two rounds. Remember the old adage, he can dish it out but cannot take it, that is my take on Hearns with all due respect to his fans.
He was stopped late eating flush rights. He was dropped for a combined something like 14 seconds in a decade by guys he went on to dominate. His second and only other loss by stoppage was when he was an old overweight man fighting Marciano… Even then it took everything and the sink.
You make some fair points, of which I agree in the main, what about this stat when assessing the chin of Louis. He was knocked down 10 times ( all in title fights ) compared to Ali who was down 3 times ( 1 in title fights ) and Holmes who was down 2/3 times from memory, Marciano 2 both title fights, Lewis 3 times , up against the above Joe,s chin is not looking that solid , is it ? stay safe buddy.
Being knocked down could be telling, the counter to that is Louis got up every time unphased for the most part. In fairness Ali had THEE chin Holmes not far behind and Marciano was iron. Lennox was poleaxed and KO'd unlike Louis ever was. I don't think he's at the front of the pack but Louis in every aspect including his chin was very good.
Michael Moorer didn't have a great chin at heavyweight. He was never KOed at light heavy though, not sure if he was down much at that weight.
Pernell Whitaker and Benny Leonard. They both resolved the issue in basically the same way- develop your skills to the point where you go a really long time without getting hit on the chin. And then, when you do, deal with it.