Easily Envy-kun's power to shapeshift that would just be beyond cool for me and that's the only power I would actually want..or maybe Lust-san's deadly claws as well xD both are from Fullmetal Alchemist por the way!
There's actually alot. One would be Shu's power from Guilty Crown (That is assuming voids and that exist.),Death the kid abilities from Soul Eater and possibly many more.
I'll like to have (From right to left) Haruhi's God like powers or Mikuru's time traveling powers or Nagato's alien powers or Itsuki's ESP powers or Kyon's! XD (I know Kyon's human, but his abilities of commenting or controlling Haruhi are amazing!) Any of them will be awesome, mostly Nagato's and Haruhi's! X3
This is also the answer to what superpower I'd want, what wish I'd have granted, and all other perguntas like that. It's just too convenient and cool! Hana hana power!