I would want to master anything related to Waterbending. Water is essential to all life, and has to power to manipulate life as well.To be honest if I was a waterbender I would probably be like Hama.
I'd like to be a water bender. With water you can heal others, and bend water within other objects (ie. blood bending, swamp bending) I personally think it's pretty awesome, but other than that I'd want earth bending. I'd love to throw rock at some people >biggrin