Air, I think. Or water, but i voted air. It's the old tvtropes issue of http://tvtropes.org/pmwiki/pmwiki.php/Main/InverseLawOfUtilityAndLethality as in, yeah maybe air wouldnt be so good in a fight, but screw that, you can run up walls and jump tens of metres at a time! and fly, that's quite cool. Ditto water, but you can't fly. But you can make any shape out of ice, or freeze people amusingly at will.
Then comes fire, it has the fewest practical applications but does look the coolest. Earth is last because although it's cool, in a civilised and policed society it would be the most strongly controlled and feared. you wouldn't be able to kick a rock at someones head in the street, because it would leave a big hole in the pavement. I have actually thought about this during my sadder moments, if there were benders in the modern world, I reckon they'd be registered and quite restricted legally, 'Heroes' style, with earthbenders the most strongly controlled, since they could take a city centre apart in minutes. Such a ridiculously good program.
Then comes fire, it has the fewest practical applications but does look the coolest. Earth is last because although it's cool, in a civilised and policed society it would be the most strongly controlled and feared. you wouldn't be able to kick a rock at someones head in the street, because it would leave a big hole in the pavement. I have actually thought about this during my sadder moments, if there were benders in the modern world, I reckon they'd be registered and quite restricted legally, 'Heroes' style, with earthbenders the most strongly controlled, since they could take a city centre apart in minutes. Such a ridiculously good program.