In a professional career that stretches back to the last century, Floyd Mayweather has been the best fighter in the world in every weight class from 130 to 154 pounds. He wasn't simply a belt-holder in those five divisions, he was the acknowledged top dog.
You can certainly nitpick his resume a little bit. I wish he has spent 2008 fighting Paul Williams and Antonio Margarito instead of moonlighting in the WWE.
Like everybody else, I wish he would have fought Manny Pacquiao sometime in the past five years.
But you don't become a 45-0, five-division world champion without winning some pretty big fights.