This is a hard question! My top two fandoms for fucking amazing, blow-me-away quality fic are Homestuck and (don't laugh) Twilight, but I don't know if I would count either of those fandoms as having "the best fanfiction" because the chaff to wheat ratio is ridiculously high in both of fandoms. I've found that in a lot of Western media fandoms where you tend to have an older fandom doing most of the writing, there's higher-quality fic overall, in the sense that there's much less badfic, but those are still mostly fandoms where I can read reams and reams of good fic without ever hitting something that's actually great, you know? Homestuck and Twilight both have (well-deserved) reputations for being overflowing with badfic, and yet. These are the canons that produce that tiny percentage of truly epic, amazing fic. Maybe that says something about the tendency of silly and/or bad canons tending to produce both the best and the worst fanfic, I dunno. But there it is.

Question: is there any canon that you would ever not want to write fic for, for whatever reason?