Judged only on Single shot stopping power , which of these guys ends fights quickest? Shaver's, Foreman, Wilder, Marciano, Cooney, Liston??
Shavers and Wilder are the true one punch guys here, guys who not only had the pop but for who a sudden KO was their best way to victory. Marciano was very strong for his size but was definitely an attrition puncher. Even the famed Walcott KO was his stealing Joe's legs and slowing him down enough to land his shot. Same goes for the Charles, Moore, Layne and even Cockell KO's. Cooney was a bit of an incomplete work so hard to judge, just didn't face (or beat) enough elite competition. Liston definitely had the pop in both hands but was more a boxer. Foreman turned the trick often, he's close. But even the Moorer KO, if you watch the rounds preceding, he is hurting Michael with heavy, heavy jabs, again getting him to a standstill where he could slip in a coup de grace. One punch power, at least against elite competition, is a rare commodity.
One punch KO artist? - yes Against elite heavy weight s (back then)-? yes Against 200 plus lb men?-no
Who did he one punch? The first Walcott was him beating the old feller to a standstill for 13 rounds so he could land his shot. The second Walcott fight was just Joe packing it in on his career. Maybe the Matthews KO which was a double hook but then Matthews was just a hype job. The Layne, Charles, Moore, Cockell, Lastarza KO's were all attrition beatdowns.
Walcott Rex Layne , pretty sure a right hand Wasn't the ending to lastarza a single crushing right hand? Yes I know he was getting shell shacked just before.
Walcott was schooling Marciano until Rocky's attrition attack, and age, could slow him down enough to get caught. You can see Walcott postholing around the ring in the latter rounds. That was the result of a continuous attack. Lots of groundwork being laid. Rex was beat to a pulp before a right hand ended it. Again, not a one-punch result but the conclusion of a steady attack. Contrast to the kind of stoppages that Wilder was getting, true one punch shots of Liakhovich, Stiverne, Szpilka, Scott, etc. He just turned off guys lights with no coordinated attack.
I suspect it would be Cooney. He was a suspect contender in a few areas but his left-hand wasn't one of them.