Here are there records of real knockouts (KO's not TKO's)and the number of fights.A couple extra fighters were added.

Foreman 28ko's in 81 fights

Frazier 10ko's in 37 fights

Marciano 26ko's in 49 fights

Shavers 63ko's in 89 fights

Dempsey 47ko's in 83 fights

Louis 36ko's in 72 fights

Tyson 16ko's in 58 fights

W Klitschko 16ko's in 50 fights

There is no real way of telling who was the hardest puncher but acording to real KO's Shavers looks to be the winner.