Materia is the best. It is so diverse, there is an infinitie number of ways you can set up your characters. There is no pre-determination involved, giving you complete freedom to make the characters in your own image, unlike FFV or FFIX. It is simple to grasp, yet incredibly tricky to master (much like playing a guitar). It is also in the game that I deem best ever (that's an opinion, not a fact). In addition, it's unique, I don' think I've ever seen anything quite like it.