As others have pointed out, how do you measure these with one another? I like @chyna‘s answer best; it was a momentous event/tragedy/turning point at the time. The Civil War was an event impressed into the memories of those who lived it, and the following generations, but today we commemorate it not with solemn observances, banquets, cemetery visits, etc., but with re-enactments by history buffs. Pearl Harbor shocked everyone in December 1941, but ten years later, it was not remembered with the same intensity that September 11 was last year, on the tenth anniversary.
Additionally, many of the things you list, @Charles, built on each other or occurred because of some of the events before them. For example, slavery was one of the central factors in the Civil War, which in turn led to Jim Crow, which led to… you get the point. You list Korea, which influenced our government’s reactions/behavior in Vietnam, which then influenced how we behaved in Grenada, Kuwait, Iraq, etc. They are all part of a national tapestry, part of our story, and as such, I don’t think there is one example that is worse (or better) than another.
However, for the sake of your question, I think one of the greatest tragedies is that we collectively failed to live up to the potential enshrined in the Constitution. We are now in the process of allowing our courts and politicians to destroy the Bill of Rights. The uniqueness of the American experiment in constitutional government influenced the political development of other societies and nations, yet we allowed our government to subvert the rights and principles outlined in the governmental framework. Many of the tragedies/disasters/failings you list came about because of how the government, in our name, abused these principles.