I think there is a very strong argument to be made that the connected history of slavery, the Civil War, Jim Crow, and ongoing systemic racism are one of the most important, and perhaps the most important and defining, aspect of American history. It impacts more people over a longer period of time than any other issue you can name, certainly more and longer than anything 2&2 listed.
The only thing that even comes close is the eradication of the American Indians. I would argue that the country has done a lot more to reckon with that legacy than it has with slavery. For example, The Battle of Little Bighorn National Monument used to be the Custer National Monument. That was changed in 2003 and a memorial to the tribes was added, along with interpretation from the native perspective. Compare that to how Southern legislatures deal with Confederate memorials. They refuse to even acknowledge the issue and insist everything has to stay as-is because "history".