My favourtie, equip Yuffie with her Conformer and power her up. Then equip her with as many counter attack materias as you can and a mastered Cover materia. Most I ever managed was 15 counters and 1 cover

Just imagine, 15x9999 = 149985 damage, every time anyone gets attacked. Just have a few exlixers/megalixers handy and you'll have no problems