Relative to the hype, Joyce has the worst chin. The supposed 'juggernaut' was destroyed by Zhang with one volley and also dropped by Chisora, who barely drops anyone.
Amongst the champions/contenders... I think I'd have to say AJ. He absolutely hates getting hit, he seems to have this look and thinking that he deserves too much respect to get hit. I suspect that Haney has a poor chin which will show in the future, however his defence is pretty good and is actually a solid boxer so he doesn't really get hit that much. British boxers seem to have weak chins: Price, Khan, Bruno, AJ, Audley.
If you had said Joyce two years ago I would have laughed at your face but his punch resistance has certainly taken its toll recently, and defo has lost some of that 'chin' he once had. He should hang those gloves up soon especially if he loses his next fight. He took some huge shots from Dubois really well IMO when he was undefeated in 2020.
Eh imo it simply acted in the worst way possible during the fight and got hit clean repetitively by what seems to be a dangerous power-puncher. Fundora on the other hand...
I'm sorry to all you AJ haters but his chin really is not as fragile as you make it out to be. I mean, Dubois is a huge puncher and AJ still got up, what, like 6 times? Wlad is a huge puncher and AJ weathered that storm even if Wlad was old as mud. Usyk rocked AJ briefly near the end of their first fight but he wasn't spaced out like Fury in the 9th. AJ gets rocked fairly easy, don't get me wrong, but he's fought some of the literal biggest punchers in boxing and eaten some fairly big shots and kept fighting. I think being an world-title level HW kinda precludes you from having one of the worst chins in boxing, it just means a less than granite chin is far likelier to get found out against the amount of big men and big punchers at HW. Anyway, the answer is former jr. flyweight champ Sivenathi Nontshinga if we're talking solely about guys at the world title level. He got KTFO by a guy with a 20% KO ratio, was KOed by Yabuki (who can punch tbf), was rocked by Suganbob once or twice iirc, and was dropped by Araneta. Nontshinga is a classic example of a glass cannon.