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Air or Wind is by far the most powerful element, simply because of the amount of things one can do with it. For starters, wind manipulation is basically telekinesis but better. And in addition, controlling all aspects of air gives access to a lot. Suffocation, the creation of vacuums, plasma and liquid nitrogen/oxygen, sound manipulation, concussive wind blasts, a nigh-impenetrable wind shield, the ability to counteract even the toughest of force fields by simply removing the air inside it, the ability to redirect practically any elemental attack, controlling the contents of the air, and even the ability to keep oneself alive by using wind to carry blood through the circulatory system and keeping the lungs and heart working normally. And then there's the ability to "see" at near-infinite distances by sensing the wind, flight, immunity to suffocation or falling/blunt force trauma, combustion, weather manipulation, and even control over the atmosphere and the ozone layer. This power is one of the few that could easily beat a time manipulator, by simply removing the air from the entire arena (except around yourself), since it seems that even in stopped time the timebender still needs to breathe. Any power can be countered (fire doesn't work if there's no oxygen, water and earth can be redirected or even controlled using wind, other projectiles can be stopped using either wind resistance or a tornado shield, electricity can be redirected using the air as insulation, ice can be melted by making the air warmer, sound doesn't work in a vacuum, any ground-based attacks can be avoided by flying, any attacks using the contents of the air are pointless since the airbender controls the contents of the air, and anything else can be blocked by either a tornado shield or by dodging using enhanced speed and strength granted by using wind as a speed boost) and an air manipulator could instantaneously destroy somebody in a dozen different ways. (That's not an exaggeration, by the way.) Even most abilities often seen as instant kills or as undefeatable take much longer than most powerful air skills, and even then most abilities can be disrupted by using air manipulation to mess with the enemy's focus. (Did you know that some hairs and a fluid in your ear are what help you balance? By messing with that using wind, you could easily cause nausea, and waterbenders would be unable to do the same trick to you, since the fluid doesn't actually contain any water.) And those are just a few things you could do with air and wind manipulation! Air/wind manipulation is, in my opinion, the most powerful element, just because it can do practically anything, and it's super useful both inside and outside of combat.

Air is by far the most powerful element as it can move water cause tornadoes (which can completely destroy the earth) and blow out fire. Fire needs oxygen and without it it will go out. Also with air you can control the movement of something. Air can also be a defensive element as it can be used for barricades.