Boxing legend Ray "Boom Boom" Mancini says Floyd Mayweather is never going to truly be "The Best Ever" ... until he steps in the ring with the most dangerous dude in the sport ... Gennady Golovkin.
Mancini -- a 2015 Hall of Fame inductee -- was outside Ecco restaurant in NYC when he sounded off about Mayweather fighting Andre Berto. Check out the clip ... Boom Boom's NOT impressed
In fact, he says it will be a "terrible" fight ... and explains exactly why. Mancini says if Floyd really wants the boxing world to kiss the ring ... he's gotta face GGG.
Interesting advice ... especially from a legend -- if you can overlook the fact Golovkin's knocked out 20 human beings in a row. Good luck, Floyd.