Old Country by Astolat
Now, it's pretty much a "classic" of the SPN/HP crossovers, so probably you guys know about it already, but I'm going to tell you why I like it:
It's an AU - going alternative after 2x22, and changing the battle of hogwarts from 1997 to 2007. Most of you know, I'm a stickler for canon (and especially a stickler for accurate timelines), so the fact that I'm recommending an AU that messes with timelines really says something about how good this fic is.
It's also a "Winchesters are actually Wizards" fic - which again, is not something I would normally go for....but they really pull it off. ESPECIALLY with Dean. I love how they found a way for Dean to be a wizard, without making him a wizard. They basically make Dean a subtle wizard - in that not even Dean realizes he's doing magic when he is doing it. He practices magic like goblins do - in craft form...and I think it's an amazingly brilliant idea, and suits the character brilliantly.
I love the fact that you still have the Sam-might-be-evil storyline - that Sam's obsession with saving Dean still leads him down dark roads.
Basically, this fic goes AU from the story, but keeps the characters true to the show.
In terms of the depiction of the Harry Potter universe: I love the insight into Hufflepuff house. The fact that they made Dean a Hufflepuff is actually true to his character, I think - loyalty above all else. I also love how the Slytherins are not cut-and-dried-evil. You still have the same questionable morality, but at the same time, they also look out for Sam.
So, yeah...if on the off chance you haven't read it. I recommend it.
ETA: I forgot to warn for SLIGHT wincest. As far as I can remember though, it's hardly noticeable...and the story is obviously really enjoyable regardless.