Sirius is easily my favourite Harry Potter character, and one of which we see so tragically little. I've never really been able to get into fanfiction, which might have assuaged some of pain surrounding my ill-focused favour, but alas.
Anyway, this one's The One Where It All Gets A Bit Dark. I do like it an awful lot, not least for that heart-wrenching scene near the end where Sirius suggests Harry could come and live with him that leaves me greeting into my hanky (or the dog, whatever's nearest).
However, my main memory associated with this is having recently rewatched the film and the horrible rendition of the "HE WAS THEIR FRIEND!" scene. Daniel Radcliffe's acting in that part is the stuff of nightmares.