Potassium is in the most reactive group of elements, the alkali metals, but it's not the most reactive metal within the group.
The alkali metals, Group 1A, are the most reactive metals because they have one valence or outer electron. They lose this electron very easily, forming ions with a charge of +1. Within the alkali metals family, reactivity increases with increasing atomic number. This makes Francium the most reactive, followed by cesium, rubidium, potassium