Tifa, just after defeating something and checking out the materia she just scored.
Actually of all the magic systems, I though FFVII was the best. Adding multiple materia to a weapon to chain spells together was a great idea. Definitely helped to streamline gameplay, something Square killed in FFVIII with the draw magic system (should have called it the drawn out system) I hope they bring the materia system back for a future FF game though.
Though FFXII was a great departure from the turn based combat of yesteryear too. It'd be nice to see something in the future that combines the materia for VII and realtime battle system of XII.