Electricity, that way I'd never need an outlet for my guitar amps ever again. I could have a an amp strapped to my back and rock out whenever I want >
. Of course, I guess it would be handy in combat or whatever, but I would be too busy thinking about all the awesome little things I could do with it. Recharge my phone at will, jumpstart a car at will, and do anything that dude from Infamous could do. Who HASN'T wanted to shoot force lightning at will?