I'd have to go for a breakout of some horrible disease. So many natural disasters can kill a person within a matter of minutes, minimizing the pain compared to something that slowly kills over the course of a few weeks. It might not lead to the same level of devastation that other natural disasters can lead to, but the degree of human suffering seems like it would be worse.
As far as other disasters:
I was in an earthquake once and was only a few miles away from the epicenter, and I even had relatives feeling it from a few states away. Definitely scary, since I really had nowhere to go when it hit, but it wasn't as bad as other things. Maybe would have been worse if I was in a country where buildings weren't built as well as they are here.
I've also been in a city that got flooded, though I lived towards the top of a rise and wasn't personally affected by it. It was sort of a shock for the community, but to most it just seemed like an annoyance, especially to the rich people who's favorite mall was closed for a couple weeks because the first floor got flooded.
Was also in a tornado once. Left quite a bit of devastation, but no one was killed. Somewhat scary when you realize what is going on. I do have family in Kansas who say they've seen a couple tornados, but they didn't seem too worried about them. Then again, they live in an area of Kansas that doesn't get hit as hard. I'd imagine their perspective would be different if they lived just a little further south (Oklahoma has gotten hit hard the last couple of years).
I've also been in a blizzard and never want to see snow again. Then again, I had to work outside during the blizzard, so my perspective is a little more negative than most people's. Still, being trapped in your house, having to brave slick roads if you can get out of your house, and likely being without power for a long time is no fun. Not to mention, the lack of power means that you'd better have good insulation in your house, or you are going to be really, really cold. It also doesn't help that it generally brings out the worst in people, but I've also worked in a retail store shortly before a blizzard, so again, my experience may be worse than most people's.
Finally, there is the dreaded hurricane. I grew up in Florida, and couldn't even escape hurricanes when I was in Virginia. They can be frightening, but I love watching rain, so it isn't all bad for me. Still, when the power goes out and there is absolutely nothing to do, it can get boring, especially when the power remains off for days after the hurricane leaves. Thankfully, unlike with blizzards, I've never been forced to work outside during a hurricane. I've also never had to evacuate from one, though I've always had a plan in case that does happen (i.e. go northwest, maybe visiting my family in Kansas while I'm at it and praying that no tornadoes decide to find me there).