Which region of the periodic table contains the most chemically active metals?

The most chemically active metals are located in the lower left-hand region of the periodic table.1

Chemical activity in metals is defined by the ease with which an atom loses its valence electrons to form a positive ion (cation). This process is governed by ionization energy, which is the energy required to remove an electron from an atom. Because ionization energy decreases as you move down a group and to the left across a period, the elements in the lower left-hand corner of the table possess the lowest ionization energies, making them the most reactive.1

The alkali metals (Group 1) are particularly notable for this reactivity. Within this group, reactivity increases as you move downward; for example, cesium loses its valence electron more readily than lithium. Consequently, these elements are rarely found in nature in their pure metallic state, as they react vigorously with water, oxygen, and other substances.21