The most popular sub-genre of Anime fanfiction (based on frequency of posts to the anime FFML and rec.arts.anime.creative). Ranma 1/2 is an anime/manga by Rumiko Takahashi.
Of course, popularity does not come without a price. Ranma 1/2 fanfiction is the singularly most loathed sub-genre of anime fanfiction. In many ways it is not unlike November Rain by Guns N' Roses (or Mmmbop by Hanson, as some particularly vehement opponents would have it.) Familiarity breeds contempt.
This hatred is not without justification. There are far too many
Ranma 1/2 self-inserts, in which an author places his
or herself in the Ranma 1/2 universe and "fixes everyone's
problems." (Often while acting out a lemon fantasy or two.) Further, the majority of Ranma 1/2 fanfiction works
are cross-overs, which many people feel are a crime against humanity, or at least against the anime fanfiction genre.
Still, there are many examples of Ranma 1/2 fanfiction with
genuine merit. Interested parties should check the
Penultimate Ranma Fanfiction Index at
http://www.rakhal.com/ranmalst.html for a comprehensive list of Ranma 1/2 fanfiction. For those interested in separating the wheat from the chaff, I suggest checking out the Ranma 1/2 Fanfiction award winners, available as a link from that page.