I'm going with earthquake. Tsunami you have a short warning if you can see the ocean. (the water recedes substantially first) which gives you a couple min to try to get to high ground. You can also simply choose to not live near the ocean. Tornadoes you can hear often before they hit. Volcanoes you have to live near. Hurricanes you also generally have time to prepare. An earthquake on the other hand, can hit hard, fast, typically with less warning then any of the others potentially have. Also you can't eliminate the risk by picking a place to live. Even places with little risk of earthquake can still potentially have a major one hit, the risk is simply quite low. After all. new faults can and do occur.
Also, potential fatality. in a tornado, tsunami, volcano you have a much higher chance of quick death. an earthquake on the other hand you have to deal with no water due to water lines broken all over the city. you could quite easily be trapped and left to die slowly of dehydration/starvation. possibly with limbs crushed and excruciating pain to deal with...
While there are volcanoes big enough to severely effect people over great distances, even eruptions like the tambora one are causing OTHER disasters. i.e. look back at the Minoan eruption from Thera at Santorini. killed a massive number of people, MOSTLY with the tsunami... Ones that are powerful enough to alter climate and cause fatalities, you can claim it's the harsh winter/short growing season killing people. i.e. cold snap/famine being the disasters.... Given the chance for one disaster to cause another one,then logically to pick between them you must pick only the initial disaster, NOT the other ones caused by it.....
