Muhammad Ali punches back at Floyd Mayweather: 'Don't you forget, I am the greatest!'

Floyd Mayweather believes he's a better boxer than Muhammad Ali - which would, if we're going by nicknames, make "Money" the greatest of all time.

Cassius Clay, 73 and suffering from Parkinson's disease, disagrees. And he let the world know Monday:

Confidence has never been an issue for Mayweather, who will be in the ring facing Manny Pacquiao on Saturday night in arguably the most anticipated boxing match in the sport's history.