FFVII Materia System.
By far and away the BEST battle mechanic I have EVER seen anywhere. The beauty of customisation, the individuality of strategy, the satisfaction of materia growth, and the certainty of outcome.

It was as perfect a system I have ever seen and I dare say if nothing else was given to me for 2.0 but a change to FFVII materia system I would be a happy bunny indeed.