Well, since this topic seems to be passably alive, and the recommendations are generally good, I don't feel too bad about tacking a couple things on.
First off, I'm surprised no one mentioned
Lesbian 1/2 by Donny Cheng, though it is rather hard to find. The fic is pretty much a hilarious take on the cliche that every anime female is a lesbian, including in this case Ranma. I'll admit it goes on a bit long (though getting through chapter 9 is definitely worth it) but the fic is pretty much comic gold.
And, while it isn't finished (but hopefully isn't dead)
Hikari no Daija by Miriani and Andrew Norris is arguably the best take on Ranma meets the Harry Potter universe in that Ranma features prominently without completely destroying the Potterverse. On the subject of HP crosses,
Brothers United by Genhoss is also quite good, with Ranma playing the role of Harry's newly found older brother. Unlike most Ranma/HP fics, Ranma still has his general badass attitude in that one.
I also personally love
A New Awakening by Jerry Unipeg and Dani Yanega, but it is far enough outside the Ranma-verse that it might not be a great pick for die hard people. Ranko in the story is next to nothing like Ranma, but its a great read for a fairy Ranma story.
Most of the others I'd mention were already listed, but the lack of Lesbian 1/2 forced me to post, it's just that important.