I'd have to say "Putting Your Heart in the Right Place" by John Biles.
I think Biles was the first one to write a *good* Ranma fanfic, and it's
managed to stand the test of time.
A close second would be Daigakusei no Ranma (DNR), which is perhaps the
longest-running continuation series. The authors have slowed down
considerably since they first began their venture, but they are still
producing episodes at a regular rate, with no plans to stop anytime
soon. Which is good, since they've managed to create a pretty nice
world for Ranma and Akane.
Then there's the infinitely complex "Ranma 2096". More authors have
contributed stories to this universe than any other, making R2096 the
most popular series to write about. :) Some of the stories have been
masterpieces. Still, my favorite moment is the end of Part 3 of the
main story, which (I am almost ashamed to admit) took me completely by
surprise. Few stories can. :)
The above is, of course, just my opinion.