Marciano's got to be up there.
"When Marciano was destroying all challengers with his blockbuster assault, the U.S. Testing Co. was asked to measure the power of Rocky's wallop. Its findings:
"Marciano's knockout blow packs more explosive energy than an armour-piercing bullet and represents as much energy as would be required to spot lift 1000 pounds one foot off the ground." Boxing Illustrated December 1963"
I can't see someone hitting much harder than that.