T(34)
The T-34 is a WWII era Soviet medium tank developed from the A-32 prototype. It formed the backbone of the Soviet armored forces of the time and had a significant impact on subsequent tank design and armored warfare.
When it was introduced, its 76mm gun and sloped armor outclassed German contemporaries, and its wide tracks and Christie suspension gave it excellent off-road capabilities. However, it's potential was hampered by an inferior crew layout, worse tactical visibility, worse reliability, and poor build quality.
Despite these issues, the Soviet Union was able to build up and maintain a mass production line throughout the war, keeping the Red Army equipped with armor despite the German invasion.
Many of its early issues were resolved in the later T-34/85 variant, which also received a larger 85mm gun to deal with the late war German Panthers and Tigers. In all, over 80,000 tanks were built over its production lifetime.