Rocky Marciano
Joe Louis
Jack Johnson
Jack Dempsey
Muhammad Ali
George Foreman
Larry Holmes
Lennox Lewis
Mike Tyson

and I cannot decide on number 10:
Joe Frazier
Evander Holyfield
Vitali Klitschko
Gene Tunney
Sonny Liston
Floyd Patterson
Ezzard Charles
Joe Walcott
Max Schmeling