Well when you get all your members I'd suggest Cloud, Yuffie, Cid or barret. With Yuffies limit break set on level 3 each of those characters hit for massive damage. That is if you have Clouds, Barrets, Cids level 4 limit. Tifa isn't bad since you can use her through out the whole game.

Anyways its all up to you. I just pick who ever deals more damage.