Quistis has both, looks, and intelligence, thus by default the most attractive female in Final Fantasy franchise, in my opinion of course. Lulu would be second, due to her strong nature, and huge chest, her strongest point by the way. Fran close third, for being overall foxy, and her accent, which has something to do with it.

As for male characters, I certainly wouldn't want to look like Genesis, Kuja, Sephiroth, Cloud, Cecil, Vaan, Luneth...androgynous in other words. Gabranth on the other hand doesn't look like a fagot, thus winner in my book, I also like his beard.