Answer
The answer is Ezzard Charles. Rocky Marciano, the undefeated heavyweight champion, faced Ezzard Charles twice during his title defenses. Their first encounter happened on June 17, 1954, resulting in a unanimous decision victory for Marciano. The rematch occurred on September 17, 1954, where Marciano once again emerged victorious, this time via an eighth-round knockout. These two title defenses against Charles solidified Marciano's dominance in the heavyweight division.