Bofors L/60 40 mm was very commonly used by the Commonwealth and the US. Germany even used captured guns, very prolific AA gun of the war.
https://en.wikipedia.org/wiki/Bofors_40_mm_gun
Twin Bofors M19 Motor Carriage, less common but it is a option that haven't been mentioned.
https://en.wikipedia.org/wiki/M19_Multiple_Gun_Motor_Carriage
2 cm Flak 30/38, very common German anti-air piece. Was also commonly mounted on a variety of vehicles.
https://en.wikipedia.org/wiki/2_cm_Flak_30/38/Flakvierling
Wirbelwind Quad 2 cm Flak 38, again less common but haven't been mentioned.
https://en.wikipedia.org/wiki/Wirbelwind
8.8 cm Flak XX is probably the most famous anti-air piece of the war, be it for other reasons then anti-air.
https://en.wikipedia.org/wiki/8.8_cm_Flak_18/36/37/41