Title. Obviously in all games there's going to be a degree of ambiguity about how you measure the best player (i.e. winrate or wins in major games or tournaments etc.). So, to flesh out the question a little, I think mafia has many more subjective aspects than many other games.
In certain communities (or even with different ppl in the same community), certain playstyles are going to be more likely to get you positive outcomes. Your personality itself may be perceived as more charismatic in different cultures and net you more influence.
One player may have a very sophisticated methodology that accurately reads a lot of players. But suppose that methodology proves incapable of ascertaining the alignment of a particular player, whereas another player's much simpler methodology does. How can we rationalize what makes the former player better at "reading" ppl, if we even can?
I also think that, even though measuring skill in all games is ambiguous, there are many more types of skill one could have in mafia, because the quirks of one's personality and thought process can have many upsides and downsides in many niche situations.
I could say more, but hopefully that demonstrates some of the directions for conversation. Imo, skill is very subjective. Your skill is to a great degree a function of the people you play with and your relationship with them. Yet, there seems to be a surprising degree of consensus about who the "good players" are, in spite of different ppl's philosophies and approaches to the game, and we view their skill as something that would also at least partially apply outside our community.