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The 44-year-old boxing legend retired undefeated in 2017 with an untarnished record of 50 wins (27 knockouts) and no losses.
During a TV appearance on ESPN, Mayweather was asked to name the best of the best that his sport has ever seen.
However, Mayweather wasted no time in revealing his top pick first and explained why he put himself at the very top of the pile.
"He's beat more world champions than any other fighter in a shorter time and less fights than any other fighter," Mayweather said, speaking about himself in the third person.
"Landed punches at the highest percentage and took less punishment. He's been world champion 18 years in five different weight divisions."
Mayweather controversially placed Ali, who is often revered as the greatest boxer of all time, in the No 5 spot.
The retired boxing superstar, who has previously claimed he was better than Ali, praised 'The Greatest' for his cultural impact for African Americans.
But Mayweather remained critical of Ali's achievements inside the squared circle and his performances against Ken Norton.
"Ali, only one weight class and really lost to Ken Norton three times," he said.
"What he did is he stood for a cause in an era when African Americans didn't stand up for their people."
Mayweather completed his selection by naming Panama's Roberto Duran second, his fellow American Pernell Whitaker third and Mexican idol Julio Cesar Chavez fourth.
He explained: "[Duran]. Came up from lightweight to world champion at 160lbs. Love that guy. A legend.
"[Whitaker]. Really, he beat Julio Cesar Chavez [the fight was a contentiously declared a draw]. And he did beat my uncle Roger twice.
"[Chavez]. Was 79-0 in different weight classes before he lost."
Mayweather also launched an extraordinary rant against Ali when he was named the best boxer of all time on ESPN's SportsCenter.
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