I think Foreman is the heaviest handed fighter in the history of the sport, but I believe Shavers was more explosive. Earnie was very heavy handed himself, but tended to put more into his punches than Foreman did. This made him a better one-punch hitter than George, but also left him more open and far less durable.
The hardest puncher lb4lb is the man in my avatar. He concussed people- quality people....and did it with one shot.