Soviet tank, arguably the single most significant weapon in determining the outcome of the Second World War (although the American 6x6 truck runs it close).
The T-34 had just started to come into service when the Germans invaded Russia in June 1941. It was fast (50 kph+, and you could take the tracks off and run on the wheels for extended road journeys), well armoured (with striking use of sloping sides) and adequately armed with a 76 mm gun (a later version used an 85 mm gun but the 76 mm versions remained the predominant model until very late in the war). It was a match for any German tank produced until late in 1942, and perversely its features lured the Germans into the strategic error of designing over-complex, costly, unreliable models to counter it. For itself it was cheap, easy to manufacture - apocryphally, during the battle for Stalingrad, T-34s were driven straight off the end of the production line into battle - and reliable, even in bad weather conditions. In the rapid reorganisation of the Red Army and Soviet arms production that followed the massive early losses of troops, equipment and territory it was adopted as the sole main battle tank, supplanting production of a wide range of other models of variable quality (one advantage of a command economy, and a striking contrast with the German's production model); annual production reached 29000 in 1944; only the American Sherman came anywhere near it for numbers. Its availability and versatility was a major factor in the Russians' final success on the Eastern Front, arguably the most important (and certainly the largest in terms of troops, equipment and casualties) theatre of operations. Late models saw service in Soviet-supplied armies up to and including the Six-Day War in 1967.
The reliable T34 chassis with its distinctive Christie suspension was also used for the SU-85 and SU-100 tank destroyers and the SU-122 assault gun.