The alkali metals become more reactive as you go down group 1. You can see this in how vigorously each one reacts with water. Lithium fizzes steadily. Sodium reacts more vigorously and melts into a ball. Potassium reacts so vigorously that its hydrogen catches fire with a lilac flame. So the order of reactivity is lithium, then sodium, then potassium, with potassium the most reactive of the three.
This pattern lets us predict the metals below potassium. Rubidium is below potassium, so it reacts more vigorously with water than potassium does. Caesium is below rubidium and its reaction with water is explosive. Francium, at the bottom of the group, would be the most reactive alkali metal of all.