1. Francium (Fr): It's the most reactive element in the entire periodic table, but its extreme radioactivity makes it difficult to study.

2. Cesium (Cs): It's the most reactive alkali metal that's reasonably stable and can be studied. It reacts violently with water, igniting spontaneously.

3. Rubidium (Rb): It's very similar to cesium in reactivity and ignites upon contact with air.

Important Note: Reactivity increases as you move down the alkali metal group in the periodic table. This is because the outer electron is further away from the nucleus, making it easier to lose and participate in chemical reactions.