Rinoa, Aerith, Tifa, and Yuna are overused in conventions, your bound to run into dozens of them but if any of them strike your fancy I say go for it!

If you have some serious time and don't mind some questionable glances, I would say Freya cause she's really overlooked and has a pretty die hard fanbase. Beatrix and Dagger are also welcomed. No one really does IX characters.

Elena of the Turks is also a welcomed sight but sadly you need a Reno or Rude to have around so people recognize who you are sadly... Hell, if you can find some guys to be Rufus and Heidegger, go for Scarlet cause I don't think I've seen someone try to cosplay as her. If you don't have the group, you might get mistaken for a sexy version of Anna from Tekken instead.

Another fun and impressive character to do is Lulu. There are not many of her for good reason but she gets serious attention if your willing to spend the time to make her belt goth dress.

VII and VIII are the easiest to do and X also gets a lot of love. Very few people do the older FFs...

I never cosplayed as an FF character though I have wanted to do a few people (I'm hoping Dissidia will make people bring out some older characters at my convention this year) but I have done a Black Trench Nobody from Kingdom Hearts (though I was technically Spike Speigel, it was a cosplay skit, you don't want to know...) and I've cosplayed as a pretty sorry looking Crono from Chrono Trigger, sorry because I don't really look like him nor was I able to spike my hair up Toriyama style.

I do have a running joke with friends that I would do a "Girls of FF" cosplay group. It stems from the fact that a good chunk of the Clouds, Sephiroths, Squalls and Turks you run into at conventions are usually done by girls crossplaying so I wanted to do an all female group done by guys.