Water. Why? Well...
Water condenses to ice and evaporates to steam.
About 75% of the earth's surface is covered by water.
I'd be able to draw moisture out of the air and clouds.
And let's not forget that the human body is about 60% water.

What do you suppose would happen if i were to freeze the water in your body? Lethal, i believe. Or maybe I just condense the water around your body into icicles, then impale you with them? I'd be able to create steps or raise a column of ice beneath me to raise me. I could create a big ball of water, put a big ice cube in it, and fly.

I'm going with water.