There are several good Fuku'ed Ranma-stories, but unfortunately most appears to die out before they reach completion.
My favourites are mentioned by others here, but I'll mention them just in case. This is listed by ranking, with the greatest to the good but not best.
Forever the Tomboy by Ranger5
Windborne by Josh Temple (no Ranma senshi, but Ranma still in fuku)
Ranko's Makeover by Josh Temple
Heir to the Empire by Ozzallos
The best of times by Ozzallos (completed)
The Earth Princess by Cloud-Dreamer
Siren's Calling by Wargiver
Princess of the Moon by C. Rose
Aftermath by Trimatter (completed)
Honor and Love by K. Hammel
Relatively Absent by Togashi Gaijin
The Return by Josh Temple
Sailor Ranko by Burgerbecky (completed)
I think that's the ones I can remember right now. I seem to remember a really great one in which Ranma was Hotaru and didn't age in girl form and she needed very hot water to change. One where 400+ years passed or something. That one was good too.