Rocky Marciano was the lineal heavyweight champion from 1952 to 1956 and was nicknamed “The Brockton Blockbuster.” His given name was Rocco Francis Marchegiano, and he represented Brockton, Massachusetts.
Marciano won the championship on September 23, 1952, by knocking out Jersey Joe Walcott in the thirteenth round. Walcott had appeared to be ahead on the scorecards, but Marciano landed a decisive right hand early in the final round. He defended the title six times before announcing his retirement in 1956.
Marciano finished his professional career with a 49–0 record, including 43 knockouts. He remains the only heavyweight champion to retire undefeated, although comparisons across eras depend on how “heavyweight champion” and “lineal” status are defined.
The alternatives are notable former champions but do not fit the clue. Floyd Patterson became champion after Marciano retired, Ezzard Charles held the title before Marciano, and Jersey Joe Walcott was the man Marciano defeated to win it.