It depends. The worst disaster can be anything. Like if someone you love dies or your house burns down. It's an opinion.
The world's worst known nuclear disaster was at Chernobyl, in the Ukraine. The Chernobyl Disaster, as it is called, happened on April 26, 1986. There is a link to a Wikipedia article on this below.
Gfth
At the moment the world's worst disaster is malaria and the mosquito that carries the virus. Although it really depends, as some people would judge i on deaths, others on property destruction
An earthquake and the tsunami. It was the 5th worst earthquake in the world. The worst disaster for Japan.
The Cyclone Mahira in 1899 was the worst natural disaster in Australia.
eastland disaster
Since you have used the word "disaster" meaning widespread destruction and distress, I would have to say the terrorist attacks on 9/11 to the World Trade Center would be considered the worst aviation disaster that led to about 2,800 deaths. The worst aviation accident, would be the Tenerife disaster was the worst to 747's ran into each other 03/27/77 there were 583 fatalities.
Chernobyl, Ukraine, 1986
No such figure will ever be known. Certainly more than half the world will be aware of the disaster.
the sinking of the MV Dona Paz, it collided with an oil tanker, and the Dona Paz caught on fire and it sank in 2 hours it happened on December 20,1987 it was the world's worst peactime maritime disaster, because the tragedy struck with the approach of christmas
There was either a hurricane or earthquake in 1900 and in 1912 ( I think) there was the san Fran earthquake -This answer is really simply an opinion, some may argue that the worst disaster was the imprisonment of the Japanese in World War 2.
Hurricane Katrina was the costliest U.S. natural disaster, and one of the deadliest. So it ranks as one of the worst U.S. disasters, but probably not the worst.