Fitz, Jackson, and Golovkin. And there is one on the horizon. Khytrov is going pro. Many who follow the amatuer game will say the decorated Khytrov hits harder than Golovkin!
Although a Rodrigo Valdez 'nut-hugger' you can't leave him out of this mix. Maybe it took two shots not one, but the combination punchers IMO can't be left out. The only guy to stop Bad Bennie, one of my fav ATG's. He rose but had his arms at his sides...GONE!
FFS, nobody ( particularly me ) has claimed Nigel Benn had a weak chin. Rather the point was that McClellan at 168 wasn't a devastating puncher, as evidenced by Benn withstanding everything Gerald threw at him, and coming back with his own shots. Again, Nigel did NOT possess a LaMotta type chin, just a fairly typical fighters whiskers.
Jock McAvoy was famous for his KO power, Hostak as mentioned, likewise SSR, Graziano & Cerdan too. there's been loads of them.
It's really tough to pick just one, as there are so many great ones to pick from. Jackson, Fitz, Langford. Golovkin is probably the hardest hitter in MW today.