How sad because it actually sounds like it could work well as a concept. Get five kids convinced to help 'improve' the planet with their destructive powers. They then both realize just how destructive their abilities are and how to help the world with them in spite of their destruction.
Anyways, I'd take water because it can be freaking broken on a planet with such massive amounts of water with life that depends on it. If not water than earth since being able to re-shape the planet would be a massive boon (and I could probably use it to find diamonds or some shit). Heart could be really potent if used on the right people but really useless if used on the wrong ones. Wind would be kinda meh. Not useless but there's better options. Fire would probably be the weakest and least useful overall; most likely to backfire for certain.
I'd rather have it so either they think they're helping the planet but are actually being used or they realize how destructive the powers are and use them for evil instead. ---