Zidane, Quina, Amarant, Eiko. I find this to be the most well balanced party with no real weakness.

Eiko is pretty weak physically, but so is Garnet and you need at least one healer. Eiko's healing spells are a lot better and her summons, though not as varied, are pretty useful.

Quina can easily do max damage with Frog Drop, and he/she has a wide variety of diverse spells with Blue Magic.

I found Amarant's Flair to have very useful skills, and he is a pretty decent attacker.

Zidane is obviously a good attacker plus I love to steal stuff.