Water Manipulation. Is the most versatile element, second to wind/air, and can greatly adapt to the environment.
Earth Manipulation. You can do tons of stuff with it such as changing the ground under someone, creating earth creatures, etc
Mana Manipulation, because you can do magic with it.
Air manipulation. Other than deep sea fights, everywhere has air. Every one needs air to breathe so the power can be lethal if necessary
What do you think?