My useless super power would be the ability to talk to and command squirrels. I mean, what good would that do? (wink wink nudge nudge)
But realistically, I'd probably choose one of the following: Immunity to aging, I wouldn't grow old, but I'd still be vulnerable to pretty much anything an average 20-something is vulnerable too, so apart from being that guy who hasn't really aged and who has a lot of stories, should a super-villian show up my best strategy would be to run as fast as I could to a volcano proof bunker... in mars... hidden under a blanket of some sort.
Either that, or I would choose the ability to read peoples minds, but to only be able to do so when that person is thinking about me. Yeah, I guess it would be kinda useful if somebody was planning on hurting me, but for most practical purposes it would only really help me figure out what people actually think of my new jacket.