I used Squall because you're forced to have him in your party, and his Renzokuken / Lion Heart is very powerful. On my first game, I had Rinoa and Irvine with Squall because I liked their characters (okay, I'm weird ).

On my second game, I often had Selphie in my party during the early parts of the game because of her Full Cure limit break. Later I occasionally had Quistis on the party because of her Blue Magic, such as Mighty Guard, Degenerator (really useful against Ruby Dragons!) and Shockwave Pulsar. The ones I used the most on my second game, though, were Rinoa (Angelo techs, usually Wishing Star) and Zell (Armageddon Fist).