16. Rocky Marciano:
Marciano is still considered one of the greatest heavyweight champions in boxing history, and for good reason. ‘The Brockton Blockbuster’ held the title from 1952 to 1956, defending the belt six times in that span. Marciano was regarded for his unbeatable mix of punching power and an iron chin.
He racked up a peerless 49-0 record before deciding to hang up the gloves at the age of only 32. When he did, he became the only heavyweight champion to retire undefeated, a record that stands to this day. Few, if any, fighters will be able to reach his unheard-of 87.76% knockout-to-win percentage. However, in a sport as violent and volatile as boxing, perhaps Marciano made the right move to retire and keep his legacy intact. He’s still one of the all-time greats of the sport. Unfortunately, Marciano passed away in a plane crash when he was only 45.