I would choose to manipulate Air. If I could do so I could pick up rock large a whole lot of concentrated air pressure under the rock and fling it away at an enemy or something. So two elements in one. There is definitely more though. Water currents can easily be manipulated by air. Strong air current can cause large waves. Three elements in one. And fire can be extinguished by Air. Pretty sure air can't manipulate fire though, if tried it would probably extinguish. Think of everything you could do. Fun time is fun.
Or I would choose teleportation power. That would be awesome. Like, gotta go to school, "Noooooo. I'm late. Teleport activate! Weeeeeeeeeeeeeeeeeeeeeeeeeeee." Yus, that would be cool. Do other stuff, too. It's limitless. :D