It would have to be the T-34. Rugged, fast, reliable, well armored, good gun, and good cross-country mobility, and most of all sheer numbers made it the tank that won the war in the East, even though it was clearly inferior to the Panther or Tiger one on one. However, the Panther was also an excellent tank, possibly the best of the war because it, like the T34, was an great all-around tank.

http://www.worldaffairsboard.com/world-wars/10410-best-tank-wwii.html
Follow the above link for hundreds of additional opinions in a survey that is taking place. MOST seem to agree with me.

Russia adopted the T-34/76 medium tank in December of 1939. This breakthrough vehicle was designed to be "shell proof" with welded 45mm frontal armor sloped at 60 degrees. It was also designed to be easy to mass-produce, maintain and repair and this proved to be perhaps its greatest advantage over German armor.

Only 1,225 T-34's had been built by the time the Germans invaded the USSR. Still, when the T-34 first went into action in June 1941 it was a nasty surprise to the German Panzers. Production was increased and the T-34 soon out numbered as well as out performed the German Mk. III and early (Ausf A-F) Mk. IV Panzer tanks that were its main adversary in the "Great Patriotic War" during the period 1941-1943. Later in the war the Mk. IV was up-gunned and the thickness of its armor increased to make it a worthy adversary for the T-34.

The T-34/76 gets its name from the 76mm main cannon with which it was equipped. This gun was perfectly satisfactory against the Panzer III and early Mk. IV, but inadequate to penetrate the frontal armor of the later German Panzer IV's (late G, H, and J's), Tiger and Panther tanks at long range. For use against enemy troops there were hull and turret mounted machine guns.

The T-34's shortcoming in main battery firepower was solved by the introduction of an 85mm main gun, mounted in a turret designed for the KV-1C tank, which made the T-34/85 one of the world's great tanks. The new gun gave the T-34/85 the punch it needed, along with overwhelming numerical superiority, to dominate the Eastern Front and drive the Germans back to the Reich. The new turret was more resistant to German shell hits, offering the Soviet crew better protection. A five-speed transmission was also adopted at this time.

The T-34 was tough, maneuverable, reliable and capable of traversing almost any type of terrain. It was, overall, the best Allied tank of the war, generally pretty comparable to the late models of the German Mk. IV Panzer. By the end of the war some 40,000 T-34's had been produced.

It remained in service far longer than any other WW II vintage tank. T34's served in Korea, where U.S. soldiers found that their sloped armor made them almost impervious to light anti-tank weapons. In fact, I understand that T34's are still in service with some minor powers, a 60 year record of service approached by no other tank.

It's not hard to understand why I think this tank is the best. Although the American M-4 is close behind. I hope I helped; have a great day!!!

The german panzer. It was a fast lighter weight and had decent armor with strong rounds.

T-34 TANK, I remember hearing about it on like the Discovery channel or something. According to Wikipedia, It is widely regarded as having been the world's best tank when the Soviet Union became involved in World War II. Soviet Union (Russians).
What I remember hearing
It was simple to build and could be spit out in great numbers.
it had an excellent balance of armour, firepower and mobility: the three factors that make up the tank.

the germans tiger 2 was the best with armour gun