Hard to say between Strange or Gandalf
Strange is the master of mystic arts, which encompasses the boundaries of the universe. He's chewed the cud with Titans and the Watcher and battled Thanos and the Surfer. He has some pretty badass powers in the end.
Gandalf is some sort of elder entity above even the elves of middle earth--created as some spirit essence that never truly can die; and with the other "wizards" as some call them, represents one type of a specific form of energy that is necessary for that world to exist. He goes from Grey to White, when a new White is needed, and you get the idea that, based on the lore, he and any of his wizard pals could single-handedly take down Sauron if they so-chose...but they don't because they have some mystical governor installed on their transmissions, and because story.