The 1986 nuclear accident in Ukraine claimed 31 lives and caused an estimated $7 billion in damage. As of 2016, the Chernobyl explosion is one of just two nuclear accidents classified by the International Nuclear Event Scale as a level 7 event; its long-term effects related to exposure are still being investigated three decades later.
It is impossible to qualify mass disasters. We can say the death toll, the land mass affected, the amount of time to rebuild, etc, but we can't state for certain that one disaster was worse than other. Chernobyl was terrible, of course, but so were the holocaust, 9/11 and the Oklahoma City Bombing.
I can name about five disasters that were super horrible. The 2010 Haiti earthquake, Hurricane Katrina, Hurricane Andrew, the Tōhoku Earthquake and Tsunami in 2011, and the Bhola Cyclone in 1970 were very bad. It all depends on what the person considers as "worst." Any situation that causes people to die in large numbers is the "worst."
Chernobyl may be a horrible nuclear power plant disaster, but we don't know the entire effects of the Fukushima plant since the Japanese tsunami and earthquake in 2011. The Indonesian tsunami of Christmas 2004 killed tens of thousands of people, and other natural disasters probably killed many more people compared to Chernobyl.