Who is each team's X factor?

You'll hear an X factor used to describe the player a team needs to be good if it wants to have a good season. I don't think that gets us all the way there, because according by that definition the X factor is just the best player on a given roster. So here's what we'll do. First, players and not groups of players or traits are X factors; an entire infield can't be an X factor. Health, also, can't be an X factor; if it could, it would be the X factor for everyone. For this exercise, an X factor is one player.