Right now top 10. If he cleans out what is a truly stacked welterweight division at the present thim he would be top 3 easy.
Top 20 or just outside of it. He is one of the best fighters to ever fight in the lighter weights but if he proves cant cut it at welterweight which is where boxing really begins he shouldn't be top 10 or even that close. Right now with the win over Cotto he can always argue that he could win at WW if he starts loosing he drops way down.
I personally think top 10 is way too high and premature. Granted, I realize my question itself is inherently unfair because Pacquiao's career is not over. For me... A win over Mayweather puts him near the top 15. Wins over Mayweather and Mosley (in 2010) would put him right around #10.