Pah, doesn't anyone listen to Iroh? Each element is equally powerful. It's how you USE the element to your advantage that makes it more powerful than the others.
And for the whole lava thing, I think waterbenders can only bend liquids with reasonable water content. Lava's mostly earth and fire, and therefore that would be earthbending and firebending.
I would choose either earth or air - earth, because there are so many branches of it (metalbending, sandbending etc) and air, because flying is just awesome >< Bending lightning would be pretty cool too, but you only use that for damaging stuff. And reeeally good airbenders - like, Toph's earthbending standard - should be able to control water, because of the whole oxygen content. Multitask ;)
