What is the most destructive kind of mass movement?

This is the reason:

Speed: Debris avalanches can travel at speeds of up to 100 km/h (62 mph), which virtually eliminates escape time.

Force: The massive amounts of rock, dirt, and water they transport have the power to instantly demolish highways, structures, and forests.

Reach: They have the ability to spread out from their starting point, impacting regions that are distant from the original slope.

Triggering effects: If they get into bodies of water, they can produce floods, dam rivers, or trigger tsunamis.

As an illustration, the 1980 eruption of Mount St. Helens caused one of the biggest debris avalanches in history, levelling everything in its path.