Actually they’re not technically heavy but medium, but anyway, medium mobile anti-aircraft guns. Which to use? Discuss.
Flakpanzer IV Mobelwagen (Armed with 37mm gun on open mount ontop of the hull – most common but least advanced of the bunch)
Wirbelwind (Armed with quad 20mm Flakvierling in large open-topped but better protected turret)
Ostwind (Like Wirbelwind except armed with single turreted 37mm flak)
Kugelblitz (practically only experimental, highly advanced, ball turret with two 30mm)
M15 Multiple Gun Motor Carriage (M3 Halftrack with large boxy turret superstructure on back housing 40mm Bofors and two .50 cal leading MGs, probably most common, but also not much better defended than M16 MMGMC)
M19 Gun Motor Carriage (M24 Chaffee hull with open turret housing dual 40mm Bofors, nearly 300 built, with twin Bofors would be the most powerful)
Crusader AA Mk I (Single Bofors 40mm in large boxy turret)
Crusader AA Mk II/III (Dual 20mm Oerliken/Polsten guns in modified standard turret; would be weakest-armed of the bunch but with its fully-enclosed and armoured turret would be the most well-protected)
Bofors 40mm SP (40mm Bofors mounted to the back of a 1/2 ton utility truck. Would be the weakest-protected of the entire bunch with no more armour than a standard truck)
ZIS-6-37or [u]ZIS-42-37[/u (Like the GAZ-AAA, simply a 37mm AA gun on the back of an unarmoured ZiS-6 truck or ZiS-42 halftrack truck (unarmoured)
ZSU-37 (Su-76’s fixed gun superstructure replaced with rotating open-topped turret housing 37mm AA gun. Historically few were built by the time the war ended and no actual action reported but who the fuck cares, it looks pretty fuckin sweet)
Germany - Wirbelwind. Quad 20mm would be hella-sweet to see blazing away at shit.
USA - M19 MGMC. Dual Bofors would likewise be sweet to see blazing away at shit.
Britannia - Crusader AA Mk II. While pretty weak (although it’d be like two German light AA vehicles), the fact that it’s fully enclosed would give it the usual British advantage as it’d be pretty much immune to all aircraft except ground attack and bombers.
Soviet Union - ZSU-37. Because I like the look of this thing.