The most realistic scariest thing I believe in is the United States will be hit with a nuclear bomb. I hope this doesn't happen obviously, but unfortunately I do think we are going to get hit by one in our lifetime. I don't know which country or terrorist group would do it, but I feel it is the inevitable. There is no way these countries are spending billions of dollars and putting this much time and effort just for fun.
Lets be realistic, that has never happen, very unlikely that it will. In fact no one has used nuclear bombs on population in almost 60 years. And every year the defense systems get better at detecting and deflecting stuff.