I personally would rate Mayweather as the greatest boxer of today (P4P King), but in my eyes there is possibly more to prove. In my eyes, to rate Mayweather the absolute greatest of today, hands down, he would need to suffer a defeat and come back from it and beat his opponent in a rematch.
For instance, lets pretend Pacman beats Cotto and a fight is arranged for Mayweather Pacman ( I personally think the Cotto fight will be very close, although I do think it will end by KO and not go the length). If Pacman beats Mayweather and Mayweather retires then I will take back alot of the praise that I have for Mayweather. Sure he will still be in the history books and I will still love him as a fighter, but for him to truly be great in my mind, he needs to bounce back after a defeat and beat his opponent. I think this will prove Mayweathers P4P status.
Pacquiao has already proved his metal, being beat by Morales and fighting him again twice and winning both rematches. For this reason I won't begrduge people there right for claiming Pacquiao as P4P Champ in there head.