That's a REALLY tough choice. Thankfully the other reccers already mentioned some of my favorites, so that made the task a little easier. I'm a Dean girl, so both these are Dean-centric:
Named (http://archiveofourown.org/series/50452) by RC_McLachlan - amazing, vivid storytelling, riveting plot with perfectly in-character Team Free Will and very cool original characters. There are scenes from this fic that I'll never forget.
The 1300 Days (http://archiveofourown.org/works/695225) by stele3 - again, incredible storytelling, great plot with some of the most interesting and original (and loveable, in one case) OCs I've ever seen. Plus lots of hurt!Dean but also competent!Dean and BAMF!Dean.
no subject
Named (http://archiveofourown.org/series/50452) by RC_McLachlan - amazing, vivid storytelling, riveting plot with perfectly in-character Team Free Will and very cool original characters. There are scenes from this fic that I'll never forget.
The 1300 Days (http://archiveofourown.org/works/695225) by stele3 - again, incredible storytelling, great plot with some of the most interesting and original (and loveable, in one case) OCs I've ever seen. Plus lots of hurt!Dean but also competent!Dean and BAMF!Dean.