Arkhangelsk said:
Most have a distinct ability that is useful. The only ones to stay away from would be Cait Sith and possibly Yuffie. I'd say the best choices would be Cid and Tifa. They're extremely powerful. Although first time I played through the entire game, I just went with whatever characters were my favorites, i.e. Vincent and Red XIII. (although now I like every character equally)
Personality wise, sure there not very appealing, but gameplay wise, cait sith can rip apart anything in the game with his limit breaks provided your lucky enough, and Yuffie is... Well, there's nothing particularly wrong with Yuffie.
Vincent is pretty good, but don't do what I did and make him a healer, then use his limit break, turning him berserk, and leaving my party helpless. Oh, and Tifa is always a good choice, as is Cid, both with amazing limit breaks.