I believe that the T-34 was the most effective tank of WWII because it was the most massed produced. No other tank was produced as much as the T-34 was. In addition to this, the T-34 was probably the most balanced tank of WWII in terms of armor, turret size, and speed. Tanks like the Tiger I and Tiger II had good armor but were too slow. The American M4 Sherman was a good tank but it was too light. The Soviet T-34 was one of the best tanks of WWII and probably the most effective.
The T-34 was good in all the ways that counted. It was very well balanced. Its sloped armor was revolutionary. It was relatively easy to manufacture. It had low ground pressure. It was mechanically reliable. Its cannon was effective, and especially so when up against Pzkw III and 4 tanks.
This tank is one of the tanks which suffered a lot of knock outs. The main advantage over the panzer 4 of the German army is that it is cheap and is able to be deployed in large quantities. The panzer 4 requires a lot more engineering and time building like other German vehicles. My criteria of effective is quality over quantity.
They were really highly mass produced and so could be provided to many fronts in large quantities. Most effective tank of WW2 was the German Köningstiger (Köningstiger was a good tank but produced by captured workers and so the quality wasn't really that high). Some German tank aces say that when a T-34 meets a Tiger, the T-34 will always raise its' hat (meaning that once it got hit by Tiger the turret flew away).
The T-34 has been voted the best tank of all time because it had innovative armor that caught the Germans off guard. It had a solid front gun as well. What wins the prize was that it was produced in such huge numbers so quickly. It could be fixed by the tank crew it self so it was a simple tank. The Most effective tank of WW2 was the King Tiger or the Tiger Royal. One might want to back step to the Tiger or Panther but none the less the Germans had the corner on the market for greatness in tanks for WW2.