Okay, I know I'm going overboard here, but I just saw the first episode of Wagaya no Oinari-sama, and I have to enthuse over Hayami Saori 早見沙織, who plays the young priestess Kou.
Not only does she do an excellent job with her simple but important role, but she elevates herself to the top of my seiyuu/singer list with her singing of the ED.
Hayami-san is only 16 and has had only one previous major role, Momoka in Touka Gettan. This role demands a similar pure, young voice, and she has it in spades. Her voice is as clean and certain as a drop of water falling in a forest pool. I would choose her over any other seiyuu for a role like this.
But her seiyuu voice is less than the half of it. She is an outstanding singer, with a sureness and musicality that are overwhelming. Her ED for Touka Gettan is something I play over and over again. And her ED for this show is just as good.
The story is that when she recorded the Touka Gettan ED, she had it pretty well down in one take, and when she emerged from the recording booth, the staff all spontaneously applauded. Columbia already seems to have signed her to a recording contract.
Besides Momoka in Touka Gettan, her other roles have been as a student in ep1 of Clannad, as Sumire in Koharu Biyori, as a student in ep12 of Ghost Hound, and as "sweetheart" in ep5 of Shigofumi.
Hayami-san works for I"m Entertainment, an agency that was rocked by a sex scandal last year and lost several seiyuu. One post on 2channel describes Hayami Saori as "I"m Entertainment's secret weapon" for recovering from the scandal.
EDIT: Her 2channel thread is not growing that quickly, but it makes up in enthusiasm what it lacks in numbers. One person said he never expected to hear such good singing in an anime song. Another said that such a skilful, fresh seiyuu voice was good to hear, but that her singing made him a fan.