I dare say I'm a sucker for good SPAG, and I believe most of the entries in this manage it fairly well. However, I think I'd argue that of the stories, the one that offers the best atmosphere is The Light That Leaves No Darkness, followed closely by Piety. The former offers a very solid foundation of the world itself, the main character and her adversary, and the sort of person you need to be to carry the metaphorical light- or indeed, the literal flame- to flush out darkness from the world. The latter, meanwhile, paints a very disturbing picture of something between a mental break and the demoniac masquerading as pure, and the consequences thereof - not to mention the uncertainty of what's about to happen to the perspective character in its ending. I think, however, that I prefer the consistency of The Light That Leaves No Darkness, given that it goes without introducing said perspective character in the last few paragraphs, and therefore that would be the one that gets my vote in this case.