I think it needs to be clarified what is meant by a single power. Something like the ability to do magic is simply a way of having more than one power, which defeats the point of the exercise.
My choice - shape changing.
1) I could assume whatever human form I want, to enjoy various sexual scenarios
2) I could be a bird and fly
3) I could be a sea creature and explore the oceans.
4) I could be a really great spy for fun and profit.
5) I could assassinate evil dictators, assume their form, and set things right in whatever country they have been abusing.
6) Most important of all - I could assume all sorts of animals shapes, and do silly things while in those shapes, in order to make my little girl laugh all day long!