The question is one of CONTROL.
Presumably therefore, all the other three elements could be made available to me, subject to Negotiation, and my control of one of the four should give me a valuable edge. .
For example, fire would be uncontrollable without water. In addition, more than 70% of creation is water-based. What would the world be, without the Oceans. I could obtain my food from the sea. As a matter of fact, all of these factors are very highly interdependent, and I could use my control of Water to negotiate for access to all of the others.
Therefore, I choose Water.