seriosly though my choice is either the .50 caliber M-2 BMG or the Bofors type AA gun.
The Ma deuce was designed before WWII, and there was not a damn thing wrong with it, they still use em on tanks, Hummers, and copters today.

The Bofors was either licensed or copied by just about every country in WWII except Japan. The US and UK used the 40mm variant, the Germans and Soviets used 37mm Bofors for anti-aircraft on ships and truck/halftrack mounted.