This is a really weird one for me. My main experience of fandom has been either in the tiniest of tiny fandoms where I'm the only person writing and I literally do not expect anyone to click let alone comment (every book fandom ever), or when I'm in a big fandom to write a tiny pairing or gen (see LOTR and Arwen/Eowyn).
Suddenly I have Sam/Dean! Jared/Jensen! What is this madness?! Luckily, I still have a real yen for rarepairs, gen and rare characters - only 11 of my 26 SPN fics are Sam/Dean.
In the end though, I picked The Full Ride Experience NC-17 because you don't get more minor than a fic which is the only one in the pairing tag. It's Ruby/Tyson Brady with background Sam/Brady and Sam/Ruby.
The concept is simple - Ruby wants to know everything about Sam, every scrap of information she can use to manipulate him and Brady has things he can tell her. They work it out through sex. It's probably the most graphic het sex I've written (apart from maybe the Gen/Jared one with the fucking machine) but it (to me) succeeds in what it set out to do with the characters - the sex is never the point, it's an instruction manual in how to manipulate Sam Winchester through it. It also helped me work out a few Ruby thoughts - specifically regarding the change in vessels and the ostensible change in personality. Also, as I've bored everyone with a number of times, my only piece of unchangeable SPN headcanon is a college fling between Sam and Brady so I liked bouncing off that.( The Seven Days )