If you can only play one Final Fantasy spin-off game, make it this one.
Compared to old Dissidia , this one got a generous amount of new characters and arenas, reworks of nearly every character (most notably, Firion and Bartz), on top of the same good old gameplay. On its own, it's still an entertaining hybrid of fighting and RPG, with a full-fledged story mode and even a "rougelike" side mode.
Duodecim looks, sounds and plays great. It's a damn shame Square Enix never did anything else with this formula.