For one to be best I think he should be pretty much perfect,
He needs to be the following
He should be living in germany or preferably where the server he is playing hosted to get low pings
Could perfectly Block all attacks from all sorts of weapons AT all times. That means he should be able to block the sword attacks from cav too.
He should be able to chamber at any given time against all type of weapon, Axes, shovels, bayonets,swords you name it and he should do it perfectly even against horse lances.
He should be able to time his attacks perfectly and be able to kill everyone with up attack or down without the fight even starts and to be honest be a master spammer able to win 10 vs 1 fight or vice versa easyly
He should have perfect footwork, he should side-step, dodge or be able to lunge or back off perfectly at his opponent. You can also use footwork to prepare situations where you can kick your opponent and he should be able to effectively do that.
Last he should have perfect eyesight, he should observe the surroundings he should know which person is at which distance and determine whether he is out of reach or not perfectly.
Now you might think what I wrote here is like superhuman but in reality we all do these things while playing the game, Its just since we are human we fail one of these and that causes us to get killed in game. So look at yourself and the players you saw in game to determine it.
TL;DR The best fighter is an ancient alien