Now that you think about it, Fire isn't really an element is it? And if so, no more than lightning is. Explanation: They aren't constant. Unlike water and earth, they are only around until they run out of fuel (Fire needs air, and lightning needs friction(?) or pressure or something in the right amounts). While water and earth will remain.
Elements are considered the basis of everything, just like the basic colours. And when fire isn't always around, it can't exactly be an element (I think).
Can someone please prove me wrong, so I wont come off quite as douchy /ignorant / stupid?
OT: I'd pick water.