I suspect Max Baer. Not everybody who squared off with Rocky rated him tops in that department. I don't recall reading any quote by any Larruper opponent saying that he was not the hardest puncher they fought. (Louis was the one common opponent of Maxie and the Rock I can think of off-hand by the way.) Was Max Baer a harder puncher than Jack Dempsey? The first knockdowns in their title winning bouts may have been the hardest punches of both, and we have that footage. No question Maxie held back after Campbell, and since all his best shots against Louis seem to have been hooks, there may be some veracity to allegations he entered that one with a hampered right. (He wiped out instigator King Levinski with a hook in their exhibition, showing both the true difference between them from their two distance bouts, as well as how dangerous his left hook was. Marciano's hook wasn't the weapon against Layne it later become against Louis and Matthews.) Concerning Dempsey's size against Willard, both Ted Lowry and Hatchetman Sheppard rated Archie Moore the hardest puncher with both hands they ever fought outright, so I wouldn't rule out Jack's power as a match for his future protege. He minimized his weight for Willard with routine fasting as well as hard training and careful dieting, while modern HWs maximize their weight in the opposite way. (Some individual HWs like Foreman and Bugner competed long enough to bridge this change in sensibilities during the course of their careers.)
Marciano. Louis could be as damaging as any heavyweight puncher with his accuracy, timing, and technique. Marciano damaged with less precise punches, brute power.
Moore also said Marciano didn't feint, but on camera we see him biting on Marciano feints. Lol But it's possible for him to honestly feel that way, as Marciano had a hard time landing clean on him, and started throwing arm punches to overwhelm with volume during the stoppage, and Moore was rolling with everything even when he was caught. Meanwhile Durrell caught an older Moore cold and clean.
I don't know about sheer power, but I don't think there's ever been a more pure puncher at heavyweight as Joe Louis, maybe Tyson is his only rival.
It's so well known that Moore said that about the first right Durelle decked him with that I guess I just took it for granted most here on Classic already knew that. Archie was telling the truth, I believe. Marciano's shots were not making that kind of sound on impact with Moore. Nor was Rocky hitting the Mongoose as cleanly, but really just beat Archie down. Jack Sharkey also described what a monstrous punch that was in the early 1980's, indicating it was the hardest shot he ever witnessed as a highly experienced referee (as opposed to being on the receiving end himself). Jack said Moore was cross-eyed on the floor, and took an admitted risk in allowing the match to continue when Archie got up.
The only common opponent who said that Louis hit harder, that I know of, was Rex Layne. Common opponents always said that Marciano hit harder, though they often went on to say that Louis was a better finisher. Max Baer, Cutis Sheppard and Rocky Marciano, are all fighters that common opponents often said hit harder than Louis. It just shows that power isnt everything.