I've never really watched the show so I'm not exactly sure on what rules "bending" follows...
If I'm a water bender can I pull water from any source? Can I change its states at will? If so then it would be pretty simple to completely turn someone into a dry mummy in seconds or literally bring their blood to a boil, or cause giant ice crystals to materialize from all directions and impale them, or just freeze them where they stand. Or heck, why not all of the above? >
If I'm an earth bender can I cause fissures to form and control things like lava at will? Can I change the state of rock at will to lava? If so then you could make any earth surface into a lake of hot lava or cause hot jets of lava to fly at people, or large rock spikes to come up from the ground, or simply raise the entire area up into the sky and drop all of it except where you are (or keep sending them up into space if you wanted...) What about creating dust around the entire area and then when the person inhales it just cause their lungs to seize up and they die?
If I'm a fire bender can I create fire at will or can I just control it? If I can set things on fire remotely what's stopping me from causing someone to spontaneously combust? Or just cause a person's clothes to burn while they're wearing them, or if I can't cause things to burn remotely, simply go with a "flame thrower" style but a sphere that encloses a person and heats the air around them so much that they burn/can't breathe. Or create fire so bright that they are blinded? Or rain down thousand of tiny balls of plasma that would simply fly right through anything they encounter....
If I'm an air bender does that cover any gases present in the air? Can I just create a vacuum around a person and watch as they gasp for air Total Recall style? How about causing their lungs to burst, or compress the air around them and have their body implode? If I can compress air what's stopping me from compressing air to the point of it becoming plasma? Or a localized jet of air that could cut like a blade and tear them to pieces or simply enter their skin and obstruct blood flow? Never mind hurricanes and tornadoes and controlling the temperature of the air....
I'm not sure about the limitations so that would obviously affect my choice, but I'm leaning towards air or water since they seem more flexible in what is possible with them...
TLDR-->If imagination is the only restriction then I would go with air or water.