Quistis is by far the hottest of the 3
and Rinoa, and then selphie.
Quistis wears interesting clothing, and has a whip (not serious about the whip thing) and her hair is good :P and in the beginning, I like a girl with glasses and a SeeD uniform xD.
Rinoa is second, she has nice long black hair and her attitude is extremely cute and flirty haha. It's funny.
Selphie is by far the cutest in terms of personality, but her hair reminds me of the mother from the brady bunch xD
Now, if we mean Battle wise.... Quistis, Selphie, then Rinoa
Quistis is with you from the beginning and I just keep shiva on her, and get the compatibility to 1000, and this proves useful throughout the whole game. Also, her blue magic tricks are pretty handy as well.
Selphie is #2 because her limit break is also pretty handy in some spots. I at least have the option to try again rather than a random dog attack.
Rinoa, is pretty useless unless I draw her some nice magic, junction her some nice stuff, and kick ass. If I don't, she's worthless.