Definitely and undeniably water. I'm not a big fan of artificial elements, prefer the primal ones.
I've spent some time carefully choosing my element.
Fire: Although it would fit my persona, I nearly always associate it with destruction. Spent a long of time wondering what can fire create, and wasn't able to come up with anything sensible on my own. Thus my rejection.
Earth: Great potential, incredible importance, although just... plain boring. Calm and stoic, unlike myself. I just couldn't even attempt associating myself with it.
Air: This one was a tough. Quick and always in motion, impossible to be witnessed with our eyes (although different sensors kick in here). It was worth noticing it, but in end I found its possibilities a tad... limited.
Water: Constantly in motion, present everywhere: under us, above us, around us and even inside ourselves. It was an only true choice for me. Either calm as a small lake somewhere in the mountains, either completely angered like a rapidity. In short, moody. More than that, since I'm generally a person of calm nature, its life-giving abilities are somewhat superior to all other elements.
And if I was able to manipulate any of given elements, water would bring me the greatest amount of possibilities. Even though I couldn't create anything static with it, it would be fine. Fleeting moments is something I've learned to appreciate long time ago already