We all have our favorite characters, but do use them in your party based on their abilities? Mine are as follows:

VII= Cloud, Barret, Tifa
VIII=Squall, Rinoa, Zell
IX=Zidane, Garnet, Quina, Freya
X=Tidus, Wakka, Lulu (although I found myself switching a lot)
XII=not vaan (haha). Ashe, Balthier, Fran.
XIII=Lightning, Sahz, Snow (although some battles require certain characters, they were my final 3)

Whilst most characters are able to learn just as much as the other characters (excluding IX), I found myself sticking to those particular characters.