In Call of Duty Advanced Warfare all of the maps are quite large, but there are some close quarter engagements. Therefore the best weapon must be an Assault Rifle since the Submachine Guns are not competitive at medium and long range compared to the best Assault Rifles. I believe the best gun in Call of Duty Ghosts is the HBRa3.
Assault Rifles are the Best Weapons in Advanced Warfare
The best strategy to do well in Call of Duty Advanced Warfare is to hold down a position that has a height advantage over the surrounding area. Being aggressive by running at enemies will mostly just get you killed due to that the maps are designed to have many paths all over the place making it impossible to cover all angles when you are moving. Submachine Guns would be good if you are aggressive, but they aren’t very good at holding down a position, especially if the area that the position is overlooking involves some long range combat.
The other weapon classes, such as Shotguns and Sniper Rifles are good in their own way, but too specialized to be the best weapon in Call of Duty Advanced Warfare.
The HBRa3 Assault Rifle is the Best Weapon in Advanced Warfare
The HBRa3 shoots the first three bullets much faster than the rest when you start firing. Those three bullets are often enough to kill enemies and they have little chance at keeping up with the amount of damage the HBRa3 does during that time.
A close contender is the AK12 which is also a very good assault rifle. However, the HBRa3 is much better at medium range than the AK12 is and, given that is the most common combat area, the HBRa3 is overall better than the AK12.
So there you have it. The best primary weapon in Call of Duty Advanced Warfare is the HBRa3 Assault Rifle.