Right now my current top fave AU's are WIP's. I don't know if that's because they are still filled with so much potential and haven't screwed up yet. I need to go back and add an AU tag to my memories.
But rosaline_kell's is rocking the VM fusion with Rebel Rebel and I'm a masochist because I hope she's not even a quarter of the way done, because I want it to go on forever.
Rereading Family Business reminded me why I had fond memories of it: the humor, Cas being weird but still BAMF, the way it's almost curtain fic but not, all the tangled emotions and miscommunications.
funkyinfishnet started an interesting world (wip) in Across Canyons. It's a slave fic, but not gross like all the others. And there's an underground railroad that the Winchester family runs, and there's pain running under the surface, and a hint at mystery with Cas.
aesc's Below Skyscrapers, I really did love the world-building. Another story where I wished there was more to it (not porn, damnit! More Victor, etc.). Heck, give me a sequel. Although it does reinforce what I hate about angelic healing: there's not much C in the H/C to do after that.
Freedom Hangs Like Heaven by aesc. It's an odd pace, a bit dry at times, but it's a fic that needs its time to slowly unravel, and there's enough hints throughout to get me through the curtains.
I reread Named for the excellent skull-squishing whump.
I'll have to reread Aurora Borealis, the The Last Unicorn fusion, but I remember that as quite lyrical and subtle and funny.
There Will Come Soft Rains by janie tangerine is decent. I'm a sucker for the "we have to share a room and now I only sleep when you are in bed with me because you keep the nightmares away". But I hated the shit with Cas's family. Basically, I liked Dean in that one.
wandersfound has two longish stories that almost do it for me.
Oh, of course, rosaline's finished Our Hell is a Good Life for it's almost-AU and Cas still being BAMF. Even though the explanation/exposition at the end did seem a bit rushed. I liked the original characters, and I'm a sucker for even the smallest of whumps having lasting effects.
Asunder, most def, by rageprufrock. The scene where Dean closes his eyes and Cas puts his hand over his eyelids? Makes my knees wibble everytime.
There might be more, I'm sure, most of them with qualifiers.
I'll link later. Shoulda been in bed hours ago.