Nic Banal, Silvestre Abainza, or Samson Libres for me. For those of you who don't know, they're all Filipino. I think it's only fair that the referee for that fight comes from a neutral country seeing as Mayweather's record with an American third man in the ring is 47-0.
Psychic Bayless because he can help tell Floyd before he get pop by PAC. PAC and Floyd can just be squaring off and Gayless would scream STOP! to PAC. We might even see RUN! to Floyd.
I think the obvious best choice would be Smoger if you want someone impartial that would not be too involved. But a fight of this importance for Floyd i would assume he would get Cortez out of retirement.