5. Floyd Mayweather
Mayweather is one of the top defensive boxers of all time. The American star built an incredible 50-0 professional record. He fought some of the greatest of his generation including Manny Pacquiao and Oscar De La Hoya. “Money” was also a shrewd businessman because he carefully selected his opponents. He fought Pacquiao when the Filipino passed his prime.
Finally, he met Conor McGregor in his 50th and final professional bout. He won by TKO against the Irishman and made a fortune in the process. Then, Mayweather turned to exhibition matches (via Sporting News). This was an easy way to milk his fans for cash. Mayweather and his team had one of the most dominant winning streaks in boxing history. It will be difficult for somebody to replicate this at such a high level.