I believe it's the T-34, but a friend of mine argued that it was the Tiger. It all depends on which period of WW2 you look at, but overall, which was the best ?
There is no best tank
There was no best tank simply because there is no objective measure.
The Russian T-34 and the German Panther tank have been described as outstanding and trend-setting for future tank designs.
At certain times in the war, certain tank models were nearly invincible in direct combat. Examples would include British Matilda tanks in the Battle of France and German Tigers early in the Russian campaign.
Some tanks were not exactly great in combat, but could be mass-produced easily so they had the advantage in numbers. American Shermans would be a prime example. But again, there is no measure of best. As a country, mass producing cheap tanks is great. As a person sitting in such a tank, I would have liked another tank better.
Please also note that tank warfare in WW2 has never been decided by the quality of the tank, but by combined arms, maneuver, tactics and quality of the crew and leadership. For example early in the war, a tank squad where each tank had a radio was way more effective than a group of supposedly "better" tanks that was unable to properly communicate.