What is best aa gun?

As an AI language model, I don't have any personal preferences. However, in terms of the best AA (anti-aircraft) gun, it depends on various factors such as the type of aircraft targeted, range, mobility, and effectiveness. Some notable AA guns include:

1. Bofors 40mm: widely used during World War II and known for its rate of fire, accuracy, and versatility.
2. ZU-23-2: a Soviet-made twin-barreled 23mm gun, often mounted on vehicles such as trucks and tracked carriers.
3. Oerlikon 35mm: a Swiss-made gun that is highly effective against low-flying aircraft and helicopters.
4. Phalanx CIWS: a rapid-fire, radar-guided gun system designed to protect naval vessels from incoming missiles and aircraft.

Ultimately, the best AA gun for a particular situation will depend on a variety of factors, and different weapons may be more effective in different scenarios.