I'll go for Foreman because I like his grilling machine,,,, no, I meant he did hit very hard, but P4P I would say Shavers.

I think Tyson's power was attached to his fist combination speed and Marciano had the "suzy q" but not one punch power and Dempsey was tough but for out and out one punch power it would have to be Foreman then a very close second is Shavers.