The leaderboards are highly inaccurate since most of them are low level boosters (not mentioning names), and leavers (still not mentioning name (DO NOT ASK). It's not based on kills because seniority rules here as some pvpers have accumulated thousands of kills over time. The best pvpers should be looked at through different leagues since a lot of skills in play style and builds are required in lower level pvp (not saying more or less that's a different debate).
Bottom line there is no way to tell who "the best" is since there are always new players in various leagues taking down opponents that would other wise be known as "the best" by those who have fought them before.
Everything I say is my opinion, it is my input and nothing more. I will not argue with those who disagree with it.
Zerious