By all acounts it's Bruno, problem is he lacked the guts and skill to use his power effectively. What is impressive from Foreman though is the fact that he lacked proper delivery system, he mostly threw arm punches, and given how much damage he did, he had as they say "heavy hands".
Hitting hard = combination of many factors that go beyond just raw strength including speed, accuracy and technique. Since Bruno has all 4 of those attributes whereas the only thing Foreman had was raw strength its possible Bruno could've struck harder. Its akin to comparing Wilder to Foreman. Yeah Wilder sure hits harder, but he seems not be as strong as Foreman was. I would bet on Foreman any day of the week in a lifting contest over Wilder or frankly any other boxer.
I think there styles give people the impression Foreman was the harder hitter- even if they were identical in power you would always most likely come away thinking George was the harder hitter per average. To me watching Bruno genuinely rock Oliver made me scratch my chin.
Foreman has very good accuracy. He would aim for the nose, kidneys, solar plexus, temple, etc with frightening precision at times and murderous intent. As for technique, he wasn't as polished or picturesque as Bruno's upright Orthodox punches, but he employed his brand of the sweet science blending brutal street brawling with old school boxing strategies he learned from Saddler and Moore.
Deontay Wilder does not have a win over a fighter that is known for having an iron chin or any real semblance of durability. So yeah his power is great but you still can easily question it, just like Shavers who's power failed him at the highest level. We aren't even certain he hits harder than Wlad (who's power is proven at the highest level). The most durable person he fought was Fury and Fury stood up to it 3 times. Even Stiverne did in the first fight, so did Jason Gavern who doesnt' have a good chin, and Duahapas did for nearly an entire fight. He gets some of these types under his belt and there are guys out there that can lend to this (Parker still, Fat Andy, Usyk, Zhang, Filip.............hell even Joyce still), I'd be more than willing to revisit my position. Until then............... Yes, yes I have.............