Melkor, by far. Sauron was his apprentice/servant, nothing more. It was only by his removal (and the estrangement of the Ainu from the world of elves and men) that Sauron could ever take power to begin with. Even so, by canon, Sauron was one of the lesser Maiar, as was Gandalf, (lesser in reference to the Valar) and he was granted more power in order to combat Sauron's evil - as powerful as those two are, they did not wield the power of the Valar, only small portions of it.