I picked Pacquiao. Since IV, he hasn't been dropped or stunned or hurt and most recently, took hellacious bombs from a much bigger Thurman right smack dab in the chin — shots that rattled iron chins like Danny and Porter. Inoue is number 2 — took 2 nuclear warheads from a much bigger, extremely motivated Donaire. Third is Clenelo. Took an uppercut from GGG that spun his head 360 degrees and had Clenelo saying hi to his mom in the seats.
GGG has a chin sculpted by the warrior Gods of Kazakhstan. Lots of other good chins about but they are mere mortals compared to GGG's.
Tiny weights shouldn't count as the power is not the same proportionally, meaning their heads are much larger in proportion and harder to accelerate for their body size and muscular power. Pac and Canelo seem like they have good chins but they are suspect PED wise. It's possible some substances increase resistance, certainly seems like it in Canelo's case, a guy who got badly hurt at a much lower weight and yet hasn't been badly troubled since, including by GGG who rocked him but not in devastating fashion. Porter got dropped by Pac in sparring. Ruiz probably has extra resistance from the extreme level of fat around his neck, face and brain. Wouldn't surprise me if he couldn't take HW shots in the miracle scenario he became a lean fighter and moved down. Golovkin has never been visibly hurt to the head. At least it hasn't been witnessed on camera or by rumour. He did seem hurt to the body in the last fight, though, which was the first time. Winner: Golovkin.
GGG looked in bad shape, though not visibly hurt, in the Deveryanchenko fight. Clenelo stops him in the rubber match.
But given Pac's diminutive dimensions (many agree that he's a lightweight competing at welterweight) and absorbing big shots from much bigger guys with little to no effects, I think he's number #1. It would be like Clenelo absorbing power shots from Beterbiev or Gvozdyk.
Golovkin and Canelo. Never touched canvas over a long stretch. Inoue seems to be there too. We will see.
Not currently fighting (but is mulling a comeback), but Ruslan Provodnikov had an iron cast chin/face. He welcomed punches to the teeth.
I guess our definition of badly hurt is different. GGG was never knocked down in the fight. Yes he was wobbled a few times and hurt to the body but was never close to being stopped. With that said I scored the fight for chenko. GGG is in his late 30's and is clearly starting to fade.