Not sure if I'd call it "The Best", but I certainly enjoyed it: Conan The Rebel, by Poul Anderson is set, mostly, in Stygia, and it describes various and diverse groups of peoples (not just one type of "Stygian" as some books will do) and places in that ancient land.
It's a good book, not only for the story, but for anyone interested in learning more about that part of Conan's world.
Also, this story takes place between the pages of Howard's story, Queen of the Black Coast. That is--Anderson sets his story after QotBC begins but before it ends. He fleshes out some of the adventuring (skipped over by Howard) that Conan does with Belit.
Anderson's not afraid to tell much of his story centered on people besides Conan, too, which is refreshing. Poul spens a lot of time with the Stygians (and more than one "type" of Stygian, too!)