Hm, it really depends on who and how you go about the sphere grid. I found that Wakka, Auron, and Khimari were the best party if you didnt wish to use summons or beat a quick, random, enemy. For the bigger battles, I usualy had Auron, Tidus ( who had haste and holy i believe) and Yuna for her summons. The party i used the most was Wakka, Yuna, and Auron