If you could only list one book you feel is a pure masterpiece, what would it be and why?

For me it would be The Children of Hurin by Tolkien. I feel like this book gets overshadowed by the LotR trilogy and nobody bats an eye at this masterpiece of a book. It is extremely dark and sets up some very major characters for Tolkien's world, all with a captivating story line that was incredibly difficult for me to put down. I never felt that type of engagement with LotR, but this one definitely latched onto me where I kept wanting more.