It depends on what you want.
Air Bending: I like the idea of flight...and for some reason they have split the spiritual side of bending with water and air.
Water Bending: Ice, Water, healing, Blood Bending. All seem quite cool. It does seem to be a more supportive/manipulating element. Though not as much as Air.
Earth: Metal, Lava, Earth all of this is quite powerful and fun. Blunt force element, but can have a very useful and practical side for "normal" living.
Fire: This to me seems to be the most powerful, but has several counters. Lightning is counter by Earth. Fire is countered by water. However, the manipulation of energy is quite interesting.
I would think I would want to be a water or air bender. I am leaning more toward water though. Interesting that Fire and Air are the only bending that you can do without the actual element present.