I don't think there really is a best party because it depends on your gameplay preferences and which abilities you've been developing. My favorite party is:

Zidane - mostly attacks, and his Trance moves are very useful
Dagger - she heals and summons
Vivi - uses black magic, casts Flare and Doomsday (if everyone in the party can nullify or absorb it)
Freya - uses Dragon's Crest and some other abilities.

Actually, my favorite team usually includes Zidane, Dagger and two out of Freya, Steiner and Vivi. Steiner's good with his physical strength and techs like Climhazzard and Shock.