Waterbending, it is by FAR the most practical power out there. Much like the question of superpowers in real life, people keep forgetting that the most powerful and impressive powers are often utterly impractical in life outside combat, and I doubt the vast majority of people would actually want to live their lives constantly in combat. That gets you killed.
Case in point, Firebending would be largely useless outside of combat, except possibly working as a living power plant. Not very interesting really.
Earthbending is slightly better as you can also be an engineer/builder, especially with metalbending. I'd actually wouldn't mind that one.
However, Waterbending allows you to HEAL. This is huge in any organized society. You are automatically in a position of power and essential to the community. The fact that you can probably heal yourself, possibly even prolong your life is huge. Want to be rich, have power etc? Be the only one that can heal a disease.
And ofcourse, waterbending itself allows you to do other things such as explore the depths of the sea, move on water, become the most capable sailor ever etc. Finally, the potential to have bloodbending is obviously huge. Suddenly you are also at the top of the combat pile, if you know when to pick your fights.
My second choice would actually be Airbending, as it is probably the second most versatile bending, due to the amazing mobility it gives you. Being able to fly is cool, and what is essentially telekinesis allows you a whole array of abilities. Plus, I would assume that were to be used with deadly intent, Airbending it the deadliest art there is sort of bloodbending. Cause... you know, exploding lungs, asphyxiating a room etc.