If I didn't have to have Zidane in my party, I'd have Amarant, Freya, Vivi and Eiko.
Amarant - Aura is great for keeping them alive. Curse is good for making Vivi's spells do more damage. No Mercy just rocks.
Freya - Two words - Dragon Knight. And Reis's Wind is good if you don't have Auto-Regen.
Vivi - Flare is such a great spell.
Eiko - Holy is my favorite spell ever. Not as good as previous versions (like FFV - it only cost 20 MP), but still a great spell. You can also do a nifty trick with her - cast Carbuncle on your party, then have Vivi cast one of his elemental spells on everyone. The spell will be reflected for even more damage.