The question is open to some speculation. The configuration of an answer in terms of analysis and support can help to make different nations as fitting the answer. I would submit that China has been susceptible to some of the worst earthquakes in the 20th Century. The three deadliest earthquakes on record all took place in China, and of the top 10 deadliest earthquakes, four of them took place in China. The death tolls in these earthquakes are staggering, with almost half a million people dying in these natural disasters. It is difficult to categorize nations' plights in suffering through natural disasters as to which one suffered more, but given the fact that other nations can be justified, I would say that the Chinese earthquakes that took place during the 20th Century would be my submission as some of the worst natural disasters on record.
I believe that the country that this question is referring to is Bangladesh. Bangladesh (which used to be part of British India and was then part of Pakistan) is in a particularly bad location geographically speaking. Its location leaves it open to being hit with terrible flooding, with violent hurricanes, and with earthquakes.
The country is particularly susceptible to flooding because it is so low in elevation. Much of the area that is regularly flooded is only a very few feet above sea level.
These disasters are particularly bad since Bangladesh is not very rich to begin with.
Post #1 correctly identifies Bangladesh as the country that experienced the worst natural disaster of 20th century in terms of number of people dead. In a cyclonic storm that hit Bangladesh in 1970 (which was East Pakistan at that time), nearly 266,000 people died. In aftermath of this cyclone many more people died of diseases. The total toll is thus estimated to be more than a million people. Another major cyclone that struck Bangladesh in 1991 killed nearly 138,00 people.
Incidentally the second biggest natural disaster of 20th century was an earthquake in Noerth-Eastern China in 1976, that left nearly 240,000 people dead.