Rikku . Mix is just imba , seriously
only with her you can play no grind game
for the sake of just finish the game ( no power level to kill big boss or big stuff) , I think everybody is useful , except Kimari yes everyone know he sux
Auron for heavy armoured fiend and tank , Yuna for summon and heal power , maybe attack with Holy later , Lulu for physical resistant stuff and elemental weaknest , Wakka and Tidus both high agile figher and good damage dealer with the overdrive and can fight underwater . Rikku can mix some Trio 999 for easy boss ownage