TBH, the most accurate way would be to have multiple sources for ranking and go with the mode choice. UFC actually does this for their pound-for-pound rankings. Even though I disagree with a few of their results, it's compiled from 46 different media panelists (plus Dana White's input) to make determinations. Unfortunately, PVP in SWTOR isn't just Team Deathmatch, or it would be a lot simpler to point at one person and say without a doubt "Soandso is the best ______. It's a fact. Your opinion doesn't matter."
A P4P ranking would be interesting and hotly debated I suspect. The pool would only be like 5 players right