Personaly if i chose one it would be ummm.... Fire! because every other element is limited to whats around the bender. but fire can be created out of nothing.
Fire could be made by warming an object or the air around you, and if you controlled that, you would be able to create fire out of everything, basically. I think that is what he was referring to, but with miswording.
I would also choose air. For more reason than one. For instance you can propel your self and be a Free Runner more easily. Also if you more air along with you body then you can pack a serious punch.
