FFI: Uhh, Princess Sara?
FFII: Maria?
FFIII: Princess Sara!! And Refia in the DS version.
FFIV: Rosa, I guess. Rydia isn't bad, either (the grown-up version, I mean).
FFV: If Butz got all 3 princesses, why can't we choose them all, too? Probably Lenna, or Cara.
FFVI: Terra, followed by Celes.
FFVII: Tifa, then Aeris, then Yuffie.
FFVIII: Rinoa.
FFIX: Garnet, aka (SPOILER)Princess Sara. Beatrix is nice, too.
FFX: Rikku. I didn't really like Yuna as a doormat.
FFX-2: Yuna. She was a lot better on this game, while Rikku had lost some of the depth she had in FFX.
FFXII: Penelo, followed by Ashe.
FFXIII: Tough call. I'd probably say Serah, then Lightning, then Vanille. That, or first Lightning, then Serah.
FFT: Agrias.

My favorite: Garnet.