An elements reactivity is dependent on wether or not the electrons in it's outermost shell are filled to their maximum capacity. The alkali metals are the most reactive of all the groups. In this group, two elements react with oxygen(Sodium and Potassium). These two elements also react with water and we know that oxygen and water are very common elements here on earth. I believe this qaulifies them as "the most reactive elements".
Edit: Another comment had mentioned Fluorine which is also highly reactive. I pulled this from the wiki.
Fluorine is a chemical element with symbol F and atomic number 9. It is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. As the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.
So obviously if it forms compounds with almost all other elements (even noble gases, which are relatively stable) it is probably "The most reactive element".