Why they rank here
The only heavyweight champion in history to retire undefeated and untied, Marciano finished a perfect 49-0 with 43 knockouts, an 87.8 percent knockout rate among the highest of any champion. Undersized for a heavyweight at under six feet and around 185 pounds, the Brockton Blockbuster overcame reach and skill deficits with relentless pressure, a granite chin, extraordinary conditioning and concussive power, most famously in the Suzie Q, his overhand right. He won the title in 1952 by knocking out Jersey Joe Walcott, whom he was losing to on the cards, with a single thunderous right hand in the 13th round, and defended it six times before walking away in 1956 at his peak, a rarity in any era. Critics note a shallow heavyweight field, but no one else combined a flawless record, a near-90 percent knockout rate, and the discipline to retire on top and stay retired.
Career profile
Each bar shows how this athlete compares with the strongest in our all-time list for that metric.