Which earthquake is the most destructive and why?

Tsunamis and earthquakes generally go together, but in general, the tsunami can be more destructive than the earthquake that causes it. A really big earthquake rattles the ground, and can cause buildings and bridges to collapse, which can be pretty bad, but in a limited area. But if the earthquake causes a major undersea landslide, the movement of the mud on the sea floor can cause a tsunami, and a tsunami wave can cause rising water on every coastline around the ocean.

The 2004 9.1 earthquake in Indonesia destroyed some buildings and killed a few dozen people on northern Sumatra. The resulting tsunami killed a half-MILLION people on Sumatra, Thailand, Sri Lanka, India, and places around the Indian ocean.

The 2012 9.3 earthquake off Honshu, Japan didn't kill anybody - but as many as 20,000 people were killed by the tsunami.