I'd probably get the most use out of air, than the other three (four if you're talking Chinese) elements.
...I should think of the elements of this world more. Wind/Water/Fire/Earth seems most common, Metal and Wood shutting out Air in classical Chinese. And then other things added in Medieval Alchemy, as I understand it.
And then there's the Periodic Table, but that's a bit much. Yeah, as far as classical goes, in my day-to-day life, I'd want air power.