Given that Zidane is mandatory, my other slots tend to be filled by any of Dagger, Eiko, Steiner, Quina, Freya and Amarant. They all have their merits and I usually rotate all those characters as I go along.
I'm an especially big fan of Dagger once she pulls her head out her ass and can spam summons. Let's face it: the reason she only knows one summon through disc 2 and is all freaked out for half of disc 3 is because she'd be so broken otherwise. Heck, look at the limitations on her White Magic pool. It's more like the spells a Red Mage would be limited to. Again, they had to find ways to keep her from being too powerful :P
Also whenever the party formation is that of a mandatory nature I play this fun little game where I leave Vivi it 0 HP constantly and tell myself that his death in the ending is actually from the neurological complications caused by being knocked unconscious so many times and for so long.