The one who has slain the most enemies with his blade and won the most battles. It is a title given to the strongest death god in each generation.
Did you really think it would kill me? I have walked across the surface of the Sun. I have witnessed events so tiny and so fast they can hardly be said to have occurred at all. But you, you're just a man. The world's smartest man poses no more threat to me than does its smartest termite.