Mine would be the Power of Hope, from the God of War series. I believe it could stomp most other superpowers. Please comment. c:
Either Speed Force, or whatever label you want to apply to Plutonian's powers in Irredeemable (subconscious subtle-reality manipulation? You get to be a Superman-type without being some OP ni-omnipotent guy...which takes some fun away from the question).
I always thought density control would be really useful and versatile.
You can become super-durable, put more force behind a punch or kick, phase through objects; make your density low enough and you could even float through the air, make it high enough and you could have a strong gravitational field.
@cooldes: Schizophrenia and psychological breaks?
Also, this is an obvious choice for me: OLDWALKER SPARK!!!!!!!!!!!!!!
I would give a lot to have Urza or Bolas' powers. The powers to create your own world/universe, and travel anywhere you want. I could visit DC or Marvel :D