I could go with worst in my lifetime for sure and I won't argue with anyone who feels it is the worst in history. It's too subjective and I can't really imagine what it was like to live through any number of past events, or periods of violence, in US history. We have quite a list.
I'll never forget that morning at work, and the whole day spent circled around a TV in the gym, wandering back to my office trying to take a stab at work, but ultimately just fixating on the TV and events, realizing that every single other work place in America was effectively shut down for exact same reason. It was unbelievable.