Foreman or Shavers. Wilder went 12 rounds with a past prime bartender named Bermaine Stiverne. Foreman blasted out Frazier and Norton in 2 rounds.
Have to pick Foreman and he did it in a stacked division in the 1970s often referred to by many as the Golden Age of Heavyweight Boxing . Then after his prime he came back from retirement after a ten year lay off at 38 years old and proceeded to win the WBA IBF belts and Lineal title to be the oldest Heavyweight Champion in history at 46 years old .
I take by hardest hitter the one punch that has the most power. I think Wilder has a better one punch KO power than the other two. So giving that these three could hit the same static target with their best punch I think Wilder would come up as the hardest hitter.
Can do the same thing for Foreman and Shavers. Foreman went 10 rounds with Levi Forte who had been stopped 12 times at that point. Shavers went 10 rounds with Vicente Rondon who had been knocked out twice in the 4 months prior to their fight.
I'd be willing to say Wilder when he is in a perfect situation, but really, when in perfect situation, each one of them blows the other guy out. What really makes a puncher special is what he does to make the punch land. Hence why Foreman eclipses the two.