I think we are lucky to have 2 current fighters who could or are already top 10 fighters of all time. I feel Pac has a headstart on Floyd already as he started at lower weights and has a better resume. floyds advamtage over Pac when it comes to all time rankings is that he hasnt been defeated yet unlike Manny.

Her are a couple of scenarios i think could mak either a real contender to be ranked as the greatest fighter of all time.

Manny Pac:
Fight and defeat Floyd Mayweather
Fight for a title at 154 and win.

If Manny did this it'd be titles at 8 different weight classes right? And he would have wins over 6 future HOF fighters. That stacks up well with anyone including Robinson.

PBF:
Fight and beat Shane Mosely
Fight and beat Manny Pac
fight and beat Sergio Martinez for the WBC 160 title.

This may sound a little farfetched but is it beyond possibility? Martinez is not really that much bigger than ODLH who Floyd has already beaten, altho he does have a massive reach advantage.
It would mean wins over 6 or 7 possible future HOF fighters, and the WBC title in 6 different weight classes. And most importantly an undefeated record. This too would stack up well against anyone in the history of boxing.