What philosopher was the most innovative?

Not to discount Socrates or Plato, but I think Immanuel Kant might have a place here somewhere. His work on metaphysics in The Critique of Pure Reason revolutionized the way Western Philosophy is done. Also, his Groundwork for the Metaphysics of Morals laid the foundation for how most people think about ethics today.

I would agree with ashby except to point out that Plato is the only way we know about Socrates' methods and so Socrates may PARTIALLY be a pen name for Plato.

Socrates. It's his method of reasoning we most often use to settle arguments or solve problems. It seems intuitive, but he defined it.