i'd say anderson silva is the best p4p fighter in the world based off all the different weight classes he's fought in. gsp is a pure WW, and has yet to fight in any other weight classes, so it's hard to make an argument for him; the exact same can be said for fedor but in the hw division. because those 2 guys haven't fought in different weight classes, how can we really say they are the best p4p?
i will admit, however, that fedor is the best fighter in the world without a question, but p4p, anderson silva takes the crown.
take a fedor and make him the same size as anderson silva, i think silva would take him.