What is the most reactive metal on the periodic table?
In theory it is francium (Fr), but experimentally it is cesium (Cs).
There is never enough francium with which to experiment because its isotopes have a very short half-life (the longest is about 22 min), and there is only about 20-30 grams in the earth's crust at any given time. http://education.jlab.org/itselemental/ele087.html
Cesium is present in large enough amounts to be studied, and experimentally it is the most reactive metal.
Reactions of the alkali metals in water.