Whoever you leveled up the most during the game.


And if you, for some reason, leveled up everybody equally, then anybody would work. I personally like Barret/Cid and Red XIII, because of their limit breaks. (Not with his final limit break though, because it sucks.) Yuffie might work too. Cait Sith is a nono, I love Tifa as a character but I'm not that big of a fan of her limit break, and Vincent is okay but I don't like how I lose control of his limit break.

If you're not depending on limit breaks, then go with whoever has the highest stats. Duh.