America is the greatest nation in the history of the world and frankly there really are no others that can even come close in comparison. That is the essence of what American exceptionalism is in a simple statement. It is not really debatable. I guess I don't understand the whole narrative about how 'evil' America is because we once had slavery. America and the United States did not invent slavery nor did we even choose to have slavery. In fact, slavery was here long before America became America. Rather it is quite the case that slavery was imposed upon Americans before they became Americans as inherited legacies of the worst of African and European societies.
American exceptionalism is not saying America is perfect but it is quite correctly stating what is frankly obvious to most clear thinking and informed people of the world. There are many good qualities to being British or Irish or Italian or French or Greek or Russian or Chinese or Japanese or Korean or Nigerian or South African or (you name it around the world). However, being American means you are from a nation which has led the world toward a much better, safer, freer, fairer, more open and caring society with equality of opportunity and without most of the ills that plague nearly all societies elsewhere. We set the example and do a dam good job of living up to our words and keeping our vows.
America is made up of humans and as such is imperfect and not infallable. Not always have we done the right thing, particularly when viewed in hindsight (often such a view is simply unreasonable of course). FDR was elected to President 4 times (hindsight quickly fixed that with an amendment ending more than two terms for anyone) and had he made better decisions in the 1930s, perhaps the country would not have suffgered for some many long years in the "Great" Depression and then through the madness of World War II. Sadly, our policies and reactions to world events are often slow and not as foresightful as they should be. But, in the end, America has responded more favorably and forcefully as a change agent for GOOD and the betterment of the lives of literally billions and billions of people. We cannot be the be all end all for everyone and everywhere but we need to try to do as much as reasonably possible to prevent depravity and brutality and inhumanity.