I feel somewhat inadequate, given that elwarre and frozen_delight and chellexxx always provide such excellent, organized masses of recs, but here are a few of my favorites:

Space epic - Tabaqui is currently writing THE WORLD WHERE YESTERNIGHT YOU DIED (up to 48K words so far) and is updating it regularly.
http://archiveofourown.org/works/1782286/chapters/3817867

In the year 2294, Jensen is an elite soldier - an ArchANGEL, known and feared through the 'verse. Jared is an indentured 'employee' of the Company, indebted for his life and his continuing health and happiness. They are both living with secrets that will cause their lives to intersect, and put them both on the path to exposing one of the biggest cover-ups in the 'verse.

Mpreg and a/b/o squick me out. That said (and even though), one of my top 5 favorites is compo67's PUNZEL 'verse (J2) - the first story alone is 96K, and it has 20 sequels. Perfect reading for a heat wave - hot sex, lots of humor, silly schmoop, plus a surprisingly intimate and realistic look at building relationships. It begins in this way: Jensen is a ride operator at the Storybook Land Canal in Disneyland. His day changes unexpectedly and he ends up outside Goofy's House with a lost little girl and a pregnant coworker.
http://archiveofourown.org/works/1106133/chapters/2225578

Another go-to comfort fic is felisblanco's THE DOORS OF TIME series (90K words). You've probably already read that one. If not, go, go!
http://archiveofourown.org/series/10091


Probably my favorite epic is A MONSTER BY ANY OTHER NAME series - over 500K words now and still being updated (regularly). It's Sam/Dean, but they aren't brothers. NOTE: It's often very dark, including torture and rape (although not terribly graphic). The same old Sam/Dean love story, with a darkfic twist. Sam grows up in a concentration camp for monsters, and Dean is raised as an only child and a hunter. Together, they make each other human.

http://archiveofourown.org/works/146982