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.
Problem with that is the large amount of AP that's required to master Ultima in order to just get a second Ultima materia.
Not my words Carol, the words of Top Gear magazine.