Here's my Disaster List in no particular order, some are obviously worse than others.
Most are survivable if we've simply got enough food/water to ride it out, but the worst are ones that last for years, forcing us to grow our own food to stay alive-
1- Chernobyl-type nuclear plant accident, but on a much bigger scale, poisoning an area as big as the USA or the whole of Europe and Asia.
2- Massive meteor or asteroid strike wiping out millions with blast and tsunami, and kicking up enough dust to black out the sun for years like what wiped out the dinosaurs.
3- World War 3 exchange of nuclear weapons poisoning virtually the entire planet.
4- A plague wiping out most humans on earth after accidental release from a bio research lab, or a deliberate release by terrorists to wipe out the population of a specific country, or perhaps a virus naturally evolves that has no cure.
5- EC Event (Economic Collapse) triggering total breakdown of law and order resulting in looting gangs etc.
6- EM pulse (from a solar flare or nuclear bomb) blows out the electricity grid and electronics, it'll be bad but hopefully the govt will have contingency plans to get relief food and medicine convoys up and running.
7- Mega-earthquake bigger than anything the earth has experienced before, demolishing a whole bunch of cities across countries or continents.
8- Mega-tsunami triggered by gigantic quake at sea, wiping out coastal areas for a hundred miles or more inland.
9- Runaway volcano that carries on spewing out dust and ash for months, throwing a grey wind-carried shroud around the planet blocking out sunlight and triggering subzero temperature drops.
10- Haywire weather (nonstop storms, tornadoes, hurricanes, blizzards, floods etc) caused by global warming, pollution, ozone depletion etc
11- Race War where armed mobs invade each others districts on killing/looting/raping sprees
