As long as I retain my mental faculties to it's fullest extent, I'd go with shapeshifting for my own purposes.
...Dude, you and I think alike
so much.
I'd be a shape-shifter and most likely I'd first try to stop local gangsters and stuff like that. Once I'd whetted my teeth, I'd go after bigger international threats.
All under anonymity, of course. The Batman method of becoming a myth that strikes fear into the hearts of the wicked would be the way to go, as opposed to the shiny "Oh, look at me I'm superman, paragon of goodness" path.
EDIT: And I don't mean the pansy-ass "you can only change into other people" or the "you can only change into things that exist" type of shape-shifting.