The two that I can think of right now are fire and water:I would choose water because it is vital to all organisms, and because it is everywhere: moisture in the earth, water in nearby organisms, even in the air, and one could easily overwhelm an opponent.I would choose fire partially because of the incendiary effects immediately associated with fire, and partially because every aspect of the ecosystem, including the planet's quality of being habitable, as well as weather systems, is greatly dependent upon the heat distribution and amount: Tweaking that could cause numerous effects.