I forgot Maskaev. Maybe he is on 2-3. But Rahman? He took about 20-30 power shots from Sanders....traded with him, and finally nailed him into the corner and won by TKO. (That Sanders who hit Wlad 2-3 Times and he was ready to get finished off)
Rahman was dropped by Sanders and was hurt numerous times. Also he was knocked out cold by a single punch from Oleg Maskaev and knocked out agan in the rematch. And then again, you can't judge the punch resisance by one fight. Maskaev, for example, was KO'ed basically every time he got hit (McCall, Tua, Johnson, Whitaker, Sanders, Peter, Aguilera) but he somehow took Rahman's punches and Lennox Lewis couldn't. So, basing on their fights against Hasim Rahman one can think Maskaev had better chin comparing to Lewis.
True, was usually fatigue and burnt up nervous energy imo that had Frank lagging as well. Obviously facing guys like Tyson, Lewis and spoon went some way to finishing him, but he didn't have a glass jaw. Only Smith actually put him down on the canvas out as far as I remember.
Only Smith put him down for a 10 count, but I don't think it's a big distinction to make when (eg) against Spoon he was slumped semi-conscious in the corner when the ref stopped it. Agree with the overall point though, that Bruno didn't have that bad a chin. I don't think his survival instincts were actually as bad as often protrayed either. Against Cummings he was able to hold and spoil until he had his legs back, with the benefit of a minute in between rounds of course. With Bruno, I think it was just a physical thing; people react differently to getting hit on the chin. You have the Marquez types who fall flat on their back eyes closed one second and jump right up eyes wide open the next with no obvious ill-effects. At the other end are fighters like Bruno, who aren't necessarily any easier to hurt, but when they're caught their body shuts down and it's all over.
The (traditional) upright style didn't help historically. There have been some notably durable British heavyweights though, such as Farr, Bugner and Mason.
I think it was Charles' extrordinarily long career fighting the world's very best fighters that wore him down. His kayo loss to Jersey Joe Walcott in 1951 was only his second kayo defeat after 11 years in the ring. The two fights with Marciano finished him off as a world-class fighter. Those fights took their toll in speed, loss of reflexes, stamina, and ability to take punishment. Gradually, he became a punching bag for the younger guys. Even then, it was the accumulative drubbings he received rather than the inability to take any one punch which led to his later knockout losses. I really don't think it is fair to count the last four kayos he suffered against him when it comes to evaluating his ability to take a punch on the chin.
Charles should not be mentioned here. I don't think he had bad chin, but Patterson was the first that came to my mind among lineal champions. Maybe Ingo was worse? Bob Fitzsimmons wasn't iron chinned at HW, but I think he's more in Charles tier than Floyd tier.