I'd control the air.
Reasons:
You can fly. (tehnically, you'd be lifting yourself together with air or something like that.
You can be invisible. Now you see me, now you dont, cos I surrounded myself with a cloud or a smoke.
You can perform telekinesis. Nope, I might not be able to perform real telekinesis, but I can sure as heck lift the entire air mass around anything I want. ha!
Shock waves. Air +air +air+ many lots of a lot of air = pressure, huge amount of pressure in a second = kaboom. If I wanted I could send anything flying away in a matter of second.
Tornadoes. Oh no, people surrounded me and are trying to get me. Too bad Im lazy and I dont feel like flying atm. I guess its over. LOL, nope, big ass tornado + air bubble which will let you stay still inside of it = WIN.
Water bending - control air around water
Fire bending - control whatever is flammable with the air and you control the fire.
Earth bending - basically same as above.
Air bending - You can control the air so there ya go!
Controlling air is multi-practical. Sure it doesnt make you invulnerable, but with a good amount of skills, you can turn this power into almost any power you want. with the right knowledge of biology, chemistry, physics, medicine etc you could probably figure out how to live in space for a certain amount of time. Or on other planets, deep caves, high mountains etc.
We are surrounded by air (referring to the air as we know it), at least here on Earth. Air is everywhere, and cos of that, it makes the most of space (right?), by controlling the air, you control the air (Palkia ftw), and if you knew enough and cared to search deeply into the space, you might just as well find the ways to control other stuff like time (Dialga ily), dimension and whatnot (Giratina ly2).
Mighzt sound a bit far fetched, but I think it wouldnt be in reality if you could control air.