I don't think I can really call any game I've played the absolute most creative, they're creative in different ways (story, music, art style, gameplay, etc)

But I'll give a nod to Ar Tonelico. On the story/music side of things, they created a functional language for it (aka no random gibberish), then made songs in that language, all of which fits into the game very naturally, and have a pretty unique style to them too.

On the gameplay side, it's mostly pretty standard RPG-ish but with the unique twist of your front-liners protecting the songstress in your party who's singing to cast magic, with successful attack/defence empowering the magic. In the third game they went a step further and made the battle music change depending on the songstress/song magic too, along with a visualization of the music and a timing mechanic to the battle system (attack just as the visualization peaks, gain more song power).

It also mixes genres a bit, a fair chunk of time is spent in the soulspace of songstresses, where the game switches to a Visual Novel presentation including choices that may or may not throw you back out to the real world.