If I had to choose, would be two powers -

- Morphing, like Mystique from Xmen. Able to change my body shape to anything I liked. Male, Female, even a sheep if I wanted to. The possibilities are endless.
- To be able to choose the moment I die. Rather like invincibility, without the whole clause where you are stuck there till the end of time. I have the option to simply "give up the ghost" if I desired.