Probably the Tsunami that swept over so many countries in Asia on December of 2004.

this would have to be one that has happened in recent times, as there is no way to calculate the economic effects of say Mt Tambora in 1815. This volcanic eruption in Indonesia was heaerd thousands of miles away, killed several hundred thousand people straight away, and the fall out did more damage to nearby populations. Moreover, the amount of ash in the upper atmosphere, blocked out the sun's light for a year, all around the world, stopping the growth of crops, and producing the worst European and North american winter ever experienced. This was precisely what lost Napoleon the war against Russia. So, if you could calculate it, I would put my money on Tambora. But the recent tsunami, with deaths over 1 million, housing, fishing fleets, land unsable for years, land lost forever, airports and ports out of commission, families broken apart, medical bills, mental health bills, etc etc would have to be at least almost as serious.

The 10 most costly natural disasters ever in the United States have all happened within the past 10 years. Many scientists believe we're entering a weather cycle that will give us even more violent storms, making the likelihood of a disaster even greater. By 2010, more people will be vulnerable in coastal areas because about 74 percent of the U.S. population will live there. Scientific evidence shows that the frequency and intensity of storms may be affected by variations in climate. For example, 1998's El Niño split the jet stream and changed normal storm patterns. On the afternoon of April 16, 1998, a series of tornadoes struck down-town Nashville, Tennessee, leaving a wide swath of damage, valued at $120 million. Because citizens were warned, they were able to take cover. Only one person died. Sixteen tornadoes tore through Arkansas and western Tennessee on March 1, 1997, killing 26, injuring hundreds, and producing an estimated $115 to $120 million in damages. On Palm Sunday, March 27, 1994, killer tornadoes left trails of devastation as they raced northeastward from north-central Alabama and northern Georgia to the Carolinas. These storms killed 42 people and injured 320. Property damage was estimated at $107 million. http://www.nws.noaa.gov/om/all-haz/all-haz7.htm

Tsunami was my first guess. As Rstarman said--Alot of damage in many different countries. Now Imagine if a earthquake was to strike off the west coast of Africa, it would send a Tsunami right to the east coast of the US. That is more destruction then any 1 hurricane or even a system of tornadoes could do.

An earthquake in Syria and Egypt devastated both countries and killed 1.1 million people.