A Midnight Clear, by teand
For no one else would this fic be considered "slice of life" but for Sam and Dean, well, saving girls and seeing angels is all in a night's work.
That sounds prosaic, but this fic is anything but. It gave me chills, and made me cry for joy, and if you know this fandom, you'll know how rare something like this is. If you are a person of Christian faith I particularly recommend this story.