I think its just because water benders are more open to change, that they have more. earth benders can't bend wood because its not mineral or mineral like but bone and glass are, as for fire I think energy would be a maybe, and air could prolly use gases.
Waterbending is much better than all of the other elements.
1. Waterbending has more subelements than any of the others.
2. I believe Water can also control Earth, because the ground usually has water within it.
3. Water is so useful in the times of most need.
i think i would have to say that i Rather Water or Fire. Though Water would have to be pretty high in training. I enjoyed Blood Bending, mostly because it made sense, to use other things as a means to functionally disable or take out enemies is always a good thing. But Fire Bending is pretty Cool too. So i guess i'm torn between the two elements.
I see all of them as mostly balanced. Although water and earth have the weaknesses of needing their element close (you can't earth bend in the middle of an ocean and you can't water bend in a dry desert). If you have a strong/good enough bender though it doesn't really matter.