This is depressing. It seems that now even the TLA fandom has been reduced to people just throwing around subjective and nonsensical arguments to support their favourites in redundant competitions the nature of who is the strongest and the like.
Seriously go watch the series again carefully and you might pick up a few important things, such as that the concept of balance is one of the pillars of the entire story. There is no element that's inherently superior to another element, there simply isn't. Most of things said in this thread are either highly subjectively interpreted or complete false.
Air not visible? It's very well visible, you can see it on every pic posted in this thread.
Airbenders being able to sense attacks because they're bald? Don't pull that out of proportion and make it sound like they developed a spidey sense that warns them from impending danger. Aang was caught off guard numerous times and he wasn't the only one.
Being able to suck the air out of someone's lungs? Zaheer used that once, on a non-bending middle-aged lady. Big whoop or was she somehow supposed to be the epitome of all the warriors in the TLA universe? That's simply not a technique that can easily be used in the middle of a fight with a bender. Likewise a firebender could easily force a fireball down your throat, an earthbender a lump of rock and a waterbender could practially waterboard you.
Airbenders can fly? There have been exactly two airbenders in the over 10 000 year history of the air nomads that could levitate. All the other airbenders could only obtain flight with the usage of their glider. All of this is convenient if you want to run, but in a straight fight its use is rather limited.
This is the truth: every element has its advantages and disadvantages, strong and weak points and none is inherently superior to the other and the same counts for the sub-elements. Granted bloodbending is really one nasty skill, however like most other sub-elements it has some severe flaws in return for its abilities. Bloodbending is an extremely rare skill and even among bloodbenders it's rare if you can use it without a full moon, so normally it could only be used once a month. And when you are bloodbending, you are also completely wide open for a counterattack as till further notice you can't blood- and waterbend at the exact same time, so you are quite vulnerable if you are not able to bloodbend all possible enemies in the vicinity. Not to mention how are you going to defend yourself when you fight people who even can retaliate while they are being bloodbent? Like Bumi who can earthbend with his eyes or Zuko, Iroh and Azula who can breath fire or characters like P'Li who can use combustion? And you consider something with those characteristics as the strongest bending art? Something that's quite marginal within the art of waterbending?
Also don't confuse the skills of the bender with the strength of the element he bends. Zaheer was already a powerful individual without being able to bend. So that when he learns airbending and uses it on a high level, doesn't mean airbending on itself is inherently that powerful. That was largely Zaheer's own skill.