Hmmm, there's so many that I would want. But I guess I should pick one.
I would want the ability to Earth Bend! I want to be able to use the earth to my advantage like Toph from Avatar or Tara from Teen Titans! It would just be so awesome to form huge rock gloves over my hands and pound my enemies to the ground with brute force! HOO HA!
Not to mention, you could form a giant rock golem to do your bidding, crushing the enemy's hideout, using it as a giant shield. There are endless possibilities!
@missy