The bottom of group 1. Francium (or Fr) is the element with the greatest metallic properties.
Francium is not a naturally-occurring element, however. It is man-made. There is an isotope of francium that exists naturally, but it's half life is so short that it decays almost instantly into a different element.
The naturally-occurring element with the highest metallic properties is cesium (or Cs), located right above francium.
Metallic characteristics decrease as you move from left to right on the periodic table.