This one's easy for me, no one ever thinks of it, but it's the best power of all... the power to alter reality at a whim. I could give myself ANY power at any time I wanted. Create villains to fight out of thin air, rule the world with the flick of my wrists... or save it from its own madness. The power to alter reality trumps all other powers. You'd be a God. You could live forever, learn the secrets of the universe, alter history... anything.
The power to alter reality.
That's always my answer to this question, and has been since I was first asked to do something like this for homework is 4th grade.
The power to kill a yak from 200 yards away... WITH MIND BULLETS!