Axel_wheel
I would go for electric powers as if mastered leads to magnetic and thermal powers. Even at it weakest, you can channel it through any conductive matter, and it is versatile. You can use it on large-scale weather manipulation, engineering feats, and enhancement on one's own biology.
My first choice would be Earthbending. Its very useful for the restoration of Moat Cailin and all other building and construction plans. Plus being able to control and rearange the environment to your desire is always a big advantage in war times. Earthbending would also be a good combo with Plant growth.