The Birth of a Nation (not looking forward to it, but it's still an important piece of film history, so it's probably worth watching for that reason)
I don't regret watching this, but I also don't think you are going to miss anything about the history of cinema if you skip it. It's obviously important. But outside of some of the battle scenes, I think you can learn everything you need of its value from all the things that have already been written extensively about it. It's not a classic I hold in terribly high regard outside of its innovations.
As for its racism, it's really really racist. The reputation is more than earned. But at the same time, as depressing as its depiction of this is, I don't see this as being a particularly arduous watch. Knowing what it was going in (essentially a trumpet for the resurgence of the KKK), I was more than prepared for what it offered and it was exactly that. It was gross and sad. But of course it was going to be that. Nothing caught me off guard in its repugnance though.
Maybe I feel this way because I already have zero expecations of how ugly people can be though, and so it is pretty hard to shock me out of my perma-despair.