My top choice would definitely have to be RpM's Ranma fic--The Pursuit of Happiness. It, along with its prequel, 'The More Things Change' was the first fanfiction I'd ever read and is still my favorite one today. I've been waiting for some six years now, but it doesn't look like RpM will be doing any more updates for it, which is horrible.

An honorable mention would also have to go to Krista Perry's 'Hearts of Ice'. I love the old fics.