Ultimate Party:

Yuna
Wakka
Rikku

Armor: Auto Haste, Ribbon, Auto-Protect and Auto-Potion.

Weapon: Celestial

Stats: All except HP are maxed (255). HP should just be enough to make everyone have 9999.

Abilities: All on the spheregrid

Start each hard battle with "Trio of 9999" and a few "Cheer's", and you've got an unbeatable party.