If you make me choose with a gun to my head I would probably say Pearl Harbor, but they are both horrific, and I think pretty equal in terrorizing and causing significant change in America. Regarding 9/11 many people died, many families destroyed, many people suffer with lingering PTSD. The after effects last for years. Pearl Harbor caused us to go to war with Japan, 9/11 with the terrorists. Pearl Harbor basically ended with us dropping atomic bombs on the Japanese, which is horrific, and the rest of the world can look at America as the one country that has pushed the button. Although, probably for the majority of Americans, having something happen in the contiguous 48 has the most impact on them because most Americans felt for many years nothing would ever happen on our soil that was so dramatic and awful. Hawaii, Alaska, Guam, Puerto Rico, even our military ships and embassies are often easily forgotten when something happens there. Way over there. Not really America or something?
It will be really interesting to see how people who are not American answer this question.