The best players are calculated by performance across all tests and XP. So generally the best players are just that, the best. But there is also an element of determination and length of play in the calculation as well because of XP and an element of strategy because of the way the classic league works.
Note: With update 1.5 we will be totally revamping the Math Legends challenges (Currently just diving digits). When this has been completed challenge progress will be included as well.