STL here. Everyone usually agrees that the mortar is the most important defensive unit, but I have to disagree. Without air defense it would be nearly impossible to defend against balloonion and mass drag. Quite simply without air defense, people would be at almost 10,000 trophies. Balloonion would be the only troop combination people use, since it would plow right through defensive bases. Now, since air defense is the most important unit, you need to know where to place it in your base. Below is an example of my base-
As you can see the air defense is triangular, and also far from the edges of the base. It needs to be spread out to be effective. Air defense also needs to be one of the first buildings upgraded at a new Town Hall since it will make you unbeatable from up top. Also, it will make your base much better in war since most people attack from the air. Make sure to get your air defense up quick to defend from balloonion and mass drag!