A tsunami is a series of ocean waves generated by sudden displacements in the sea floor, landslides, or volcanic activity. In the deep ocean, the tsunami wave may only be a few inches high. The tsunami wave may come gently ashore or may increase in height to become a fast moving wall of turbulent water several meters high.
Are they the worst disaster? No, I don't think so. They are only the effects of a larger natural occurrences. I personally would believe that earthquakes and volcanic activity present a more serious threat. This doesn't mean that tsunamis aren't dangerous. They are quite destructive.