What is the most reactive group of metals?

The Group (I) , Alkali metals, are the most reactive.

In Water : -

Lithium ; gently fizzes/

Sodium ; fizzes and spits

Potassium ; spits and bangs

Rubidium ; bangs nd explodes

Caesium ; is Explosive.

Francium ; is Radio-Active, so dissolving in water is NOT done. because of the spread of radio-active material.