Its a hard choice to make, considering there are multiple things you could do with one of these elements. You also need to know to properly control them too because either one of them can be dangerous if not careful.
I chose air though, because its just something basic and there are cool things I could do with air. I could interact with already fluctuating currents(natural wind, wind from my hands, a hand fan, air conditioning, electric fan etc). I could even conjure my own outside.
Also, I could put out fires or make them stronger.
Note: I would use my power wisely.