It's impossible to put a number on him because there are many greats from the past but all I need to know is he's the best from this era...the fighter of the decade winning fighter of the year 3 times and achieving championship status in 8 divisions.
I wouldn't rank Floyd top 15-20 if he doesn't face Pac. I'd rank him somewhere in the low 30's. But if he beats Pac I'd probably put him in the low 20's, so it would be jump of about 10 spots, which on an ATG p4p list is huge. Similarly if Pac wins I'd give him about a 10 spot jump as well.
He is the all time number 1. He is a boxing icon, a living legend, a true humble guy. There will never be another boxer like him who is great inside and outside of the ring. If Pacs mind is on the fight and his opponent stays off the PEDs then he cannot be beaten. He is a machine in his prime, unbeatable. When he whoops floyd he will own the number 1 rank forever.
Top 10 easily If Floyd didn't duck him when they were both in their primes manny would have had an argument for number 1