Nobody agrees he is the best of all time. Nobody. Except people that don't know boxing.
Floyd is probably in the top 20 boxers of all time, but just at the edge - almost everyone in boxing would rank Sugar Ray Robinson, Joe Louis, Muhammad Ali, Joe Frazier, Marvin Hagler, Henry Armstrong, Stanley Ketchel, Roberto Duran, Jack Dempsey, Sugar Ray Leonard, Willie Pep, and several others above him.
And to be fair to Floyd, he has fought some good opponents, especially at lightweight - Castillo and Corrales come to mind, as well as a few others. He fought Hatton when he was undefeated, he fought Oscar before he was totally shot, he fought Shane Mosley coming off the Margarito win. Alvarez is certainly a huge feather in his cap as well.
That being said, he is much better than the current competition. That's not his fault, but it will hurt his all-time rank just like it's hurt Rocky Marciano's.
I probably rank Floyd somewhere in the top 25 of all time but nowhere near the top.