Studying and understanding the worst that nature can throw at us is one of the most interesting parts of being an Earth scientist.
Defining 'worst' is, of course, subjective, and leads to a choice over whether the number of deaths, or $$ property damage, is the statistic to use. Here is a representative collection of some of the 'big ones', organized in more or less inverse chronological order.
1. The October 8, 2005
magnitude 7.6
earthquake in Pakistan was not especially large, but the more than 40,000 victims has raised it to the level of a major catastrophe. Without doubt, however, the most devastating loss of life in recent years was the much larger 9.3 Sumatra-Andaman earthquake (the third largest ever recorded) / tsunami in late 2004 that is now estimated to have claimed 275,000 lives. The tsunami was responsible for the bulk of the damage and casualties. The total costs of these events are still to early to measure, but probably won't exceed the property damage caused by the 1995 Kobe earthquake, now estimated at more than $150 billion. Within the US (excluding earthquake-prone Alaska), neither the Los Angeles Quake (1994) nor even the great San Francisco event of 1906, were nearly as damaging.
2. Hurricane Katrina, however, was a major international story. It struck the vulnerable US Gulf Coast in August 2005 and brings the still-rising death count to over 1,000, which is serious but not remarkable for a major disaster. But together with extensive urban flooding that was a secondary effect, damage estimates from insurance costs alone are at $30 billion, with total rebuilding likely to exceed Kobe. This is easily the most expensive disaster ever to hit the US, eclipsing Andrew in 1992. Interestingly both hurricanes landed twice, first in Florida, then in Louisiana. From Andrew the death toll was 'only' 26, but the property damage added up to (what was then) a staggering $25 billion.
3. The list contains several volcanoes. That of the Nevado del Ruiz (Columbia) in 1985 ended the lives of 25,000 people, most of them caught in a massive mudflow that poured down the stricken mountain. By comparison the Mount St. Helens eruption (1980) shattered the peak but had few victims.
4. The most devastating earthquake in modern times was the famous 1976 Tangshan magnitude 8 event in China, whose toll varies between the official 255,000, and a possible 655,000. This event truly began the modern era of intense seismic hazard monitoring in China and the West. Little is known of an earlier lethal earthquake that struck the Chinese city of Shaanzi in 1556. No magnitudes are quoted, and of course no recordings exist, but it is said to have taken the lives of 830,000 people.
5. This choice again highlights volcano-related disasters. Should I choose the Tambora, Indonesia volcano of 1815, in which 80,000 people died of the subsequent famine, or the famous Krakatoa explosion, again in Indonesia, in 1883 in which more than 50,000 people perished, many of them like Sumatra engulfed in a tsunami? Well, you see I did both!
6. Very close to home here, the New Madrid earthquakes of 1811-12 in southern Missouri remain the largest (3) earthquakes ever to hit the contiguous U.S. The main event is now estimated at a magnitude 7.8, although some earlier reports placed it higher (>8). Damage was relatively light due to the sparse population at that time in the Mid-West. Not so if it would happen today!