I heard from a friend that psychs used to ask this question all the time, because the super power you choose is usually the one thing you want most (i.e. shape-shifting--being able to become someone other than yourself because you're uncomfortable about who you are, being invisible--being able to escpae, to get away, .... etc.)
I would be able to fly. Preferably with the help of a pair of extremely large, raven-feathered wings. To get away, be free, escape...