Hey there, I very recently answered a question like this. You may be interested in that answer!

The short answer is that the performance of the tanks of all nations was relatively solid, with edges going in certain sectors to one or the other. It truly came down to doctrinal employment, soft factors such as crew training , cohesion and rate of crew replenishment (all of these deteriorating throughout the war for the British and Germans). Tangential to how they were used was often where they were used. Naturally, being on the strategic offensive from 1943-45 meant that the Allies often had to suffer all the disadvantages of attacking a prepared foe, often in very poor tank terrain. This all has impact on how a tank design would perform.

You really need to define effective. What makes a tank effective? You have so many factors, quality of the crew, quality of the workmanship of the tank, ability of the gun to penetrate the armor of comparable tanks, mobility, reliability and lastly and probably far from least the quantity. WWII was also a somewhat long war and the answer IMO changes as the war goes on.

In the "Beginning" of the war 1939 until 1941 Germany without a doubt had the most effective overall armored force. They pioneered tactics, they had tanks in quantity that were good enough to defeat the smaller numbers of Allied tanks they encountered at any given time and the crew training was good. Some would argue that many of the allied tanks were actually superior 1 on 1 in the early stage of the war to their German counterparts, the bottom line is the allies lacked the tactics and training to make use of any material advantages they may have had.

In the "middle" of the War, lets called it 1942-1943 would argue that the United States produced tanks had a slight edge. The newer M4A1 Sherman tanks were arriving in mass with a new gun, the crew training was acceptable and the tactics were beginning to catch up to the Germans.

During this time period we saw the iconic German tanks beginning to show up en mass, the Tiger and the Panther though these tanks did not show up in great enough number or with acceptable reliability to IMO tip the scales in favor of the Germans. Towards the end of this period, German craftsmenship due to material shortages began to slip.

During this period as well the Soviets were beginning to field the T-34 in larger numbers, IMO it was still not in the field in sufficient numbers to be crowned "the most effective.

Late war 1944-1945 I think the clear winner would have to be the Soviets. The T-34/85 IMO was the overall "finest" tank of the war given its excellent firepower and more than acceptable protection for the crew due to its sloped armor. Along with the IS-2 heavy tank and numerous increasingly effective tank destroyers this gave the soviets the edge. IMO again, their tactics were never quite the equal on grace of the Germans or daring of the Americans, but that is more due to the differences in how the various armies fought and cultural differences regarding the value of their own mens lives.

On the German side Material shortages were leading to German tanks with more and more reliability issues. While the armor on many tanks such as the Tiger and Tiger II was thick enough to stop the shots from many allied tanks due to a lack of needed alloys the armor was often of inferior quality and cracked or caused spalling which could injure/kill the crew. Nor could those tanks and crews be replaced at the rate at which they were being depleted.

On the US Side the Iconic "Easy 8" Sherman was reaching the front in greater numbers giving them a more effective gun on their main tanks, logistics and production were excellent and workmanship was still fine. You also saw increasing numbers of newer tank Destroyers such as the hellcat.

Lastly, on the UK side had an excellent tank in the Cromwell by the end of the war but on sheer weight of production it could not compare to the Shermans being put out by the US. The "Firerfly" was a good variant of the Sherman employed by the British but it was produced to the same scale as the Sherman or T-34