The Sycamore Lady
Honestly, it's probably his best story.
Did you ever read the "Turin Turambar" chapter of the Silmarillion? It's the same story, although admittedly not as fully fleshed out. It's a shame he never lived to publish it personally - however fantastic an editor Christopher Tolkien is, I still I wish I knew what version Tolkien himself would have preferred.
Anyway, heartwreching story, and I love it. I love the tragedy, and the humanism, and the slaying of the dragon, and all the norse/anglo-saxon narrative and thematic references, and everything. It's fantastic.
Unfortunately he never lived to finish the chapters in The Silmarillion.
In the introduction to "The Book of Lost Tales", Christopher Tolkien talks about how The Silmarillion is a hard read since it's not told like a normal novel (in which you have a central narrator), that he wanted to go back to the earlier notes (which is where the 2 books of lost tales came from)...
That said, Children of Hurin is an amazing read. But if you've read the Silmarillion, then you already know the story.