I haven't played all the Final Fantasy's so I don't know all the characters. (I haven't played 3, 4 or 5)

Front Row - Auron Steiner Kiros
Middle - Vivi Vincent
Back - Aeris Yuna Barret

Auron
Reason: Because he has high health, and has overwhelming power. He can also use Power break etc...

Steiner
Reason: Because he is powerful, and has a good trance. I also joined him with Vivi so that he can do magic sword powers.

Kiros
Reason: Because he is sort of like a male version of Rikku. He has great stamina.

Vivi
Reason: Its Vivi, he's probably the best charcter square has ever come up with. He has very powerful magic power, and can make Steiner very powerful with the magic sword.

Vincent
Reason: I thinl Vincent is so cool, because he is dark, and has a mystery about him. He has a good ranged attack, and can morph into powerful monsters

Aeris
Reason: Because she can cure her party, and she has lots of healing limits, including Healing Wind.

Yuna
Reason: She has an assortment of healing spells, and can summon powerful aeons.

Barret
Reason: He has a cool gun arm. He has a powerful ranged attack.