The longest-reigning world title-holder in boxing history, Calzaghe ruled the super-middleweight division for two weeks short of 11 years after defeating Chris Eubank on points for the vacant WBO title in 1997.
He destroyed Jeff Lacy in a career-defining unification showdown in 2006, which he followed with a superb victory over Mikkel Kessler.
He then travelled to the United States to outpoint American greats Bernard Hopkins and Roy Jones Jr at light-heavyweight.
"He is the best, without a doubt," Warren told Radio 5 Live's Sportsweek show.
*Source; BBC SPORT*
He destroyed Jeff Lacy in a career-defining unification showdown in 2006, which he followed with a superb victory over Mikkel Kessler.
He then travelled to the United States to outpoint American greats Bernard Hopkins and Roy Jones Jr at light-heavyweight.
"He is the best, without a doubt," Warren told Radio 5 Live's Sportsweek show.
*Source; BBC SPORT*
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