Which alkali metal elements is most reactive?

Cs. As you travel down group 1A of the periodic table (alkali metals), reactivity increases. Cesium is the lowest one on the column within the context of the elements you specified, so it's the most reactive. Also, if you look at the electron configuration of cesium, it has one unpaired electron way out that it really wants to lose so it can look like a noble gas, making it very reactive.