Some people think its francium and some people believe it is either sodium or potassium.
Some people believe that the most reactive element is francium because the larger the atom, the easier it is to get rid of the valence electrons and the easier it is to react with other atoms. so now if you look at the periodic table, francium has the largest radius and so that's why some people think its francium. but in my opinion, Francium AND Cesium.