I'll go for something as unglamorous as an artillery piece - the trusty Soviet 76.2mm gun. Artillery inflicted the majority of casualties during WWII, and the 76.2mm (in various models) was prolific on the eastern front.

Many versions also doubled as AT (and even AA ) guns with good performance. Cheap, easy to transport, low maintenance, etc, etc.

__________________
"Nervous wait for the whistle to blow
Rush of blood and over we go..."
- Iron Maiden, "Paschendale"