Sneak attack and counter (not counter attack) materia are good too. And get slash-all materia. If you upgrade it, the "slash-all" command will turn into "Flash" which is an instant kill attack on all enemies. However you should remove it before fighting bosses as it will have no effect. 4xCut and Quadra Magic are a must too.

Heres a tip: equip these materia combinations: Ultima=Counter, Ultima=HP Absorb. With this, whatever character you equipped these to will counter with Ultima when attacked and absorb HP at the same time. If you equip another Ultima with MP absorb as well then you can also regain lost MP from the use of Ultima.