What are the most reactive metals?
The alkalimetals (group I of the periodic system).
All alkali metals have only one electron in their outer shell.
They 'want' to get rid of this to form a noble gas formation.
Since the positive nucleus still attracts even this maverick electron, it's easy to see that the farther away from the nucleus, the less attraction.
So we expect reactivity to go up while we move down the column of the periodic system.
When reacting with water, the electron goes to one of the hydrogen atoms in water to create (explosive) hydrogen gas.
You may want to see this video: