Wladimir Klitschko is today's heaviest hitter... Dr Steelhammer is at 80.65%.

Historically you have Rocky Marciano, Sonny Liston, George Foreman, Ernie Shavers, Mike Tyson representing the HW division of super hitters then you have the little guys like Carlos Zarate with 90% KO wins, Wilfredo Gomez 87.5%, Khoasai Galaxy 86% , then there is Aaron Pryor at 87.5% and heavy hitters like Tommy Hearns 71%, Felix Trinidad 77%, Julian Jackson 80%.

I think any fighter who has a KO winning percentage greater than 70% is well up there in the heavy hitter department.