Got to go with Keyblade transformations. Not enough to break the game yet the variety makes it a versatile mechanic and doesn't require an equalizer like a drive meter or Anti Sora. You can choose when and if you want to activate it, and switch Keyblades to keep the transformation in reserve for when you need it. Visually amazing as well, like Crystal Snow's second form finisher and Wheel of Fate gives you extra range with your Keyblade swipes and jabs. Being able to summon watery Kraken tentacles is even better. The fact you don't need to level them up, eliminating grinding, the icing on the cake.