Hmm, it's a tie between omniscience and shape-changing.
Omniscience - I'd love to know everything, just to be that knowledgeable, but also to know what people are really thinking when they say or do things. I'd especially like to know how to easily get lots of money for myself.
Shape-changing would allow me to be beautiful. As the Jet song goes, "with a face like that, you don't need money, do ya, honey?"