Best tank 1v1? Most "combat effectiveness" per ton? Or unit cost?
Do we factor in the country's ability to actually produce it? For example, some tanks were quite competent, but were produced late, and in small numbers.
Do we adjust for the ability to actually get to the battlefield in the first place? WW2 tanks were notorious for breaking down on the way to battle (or at any other time), requiring heavy tank transporters, or being hard to repair in the field.
Regardless, it's fun to debate.
My personal view of the effectiveness of a tank, or any other piece of machinery, is it's fitness for the purpose for which it was eventually used.
The Sherman? Looking at the range of models over it's period of service in WW2, it lacked the "hard" combat power that translates well into tables (and computer games). Armor thickness, gun calibre, etc. But as others have said, it was designed for mobility, and the ability to be deployed and sustained on the other side of the planet. It's design prioritised ease of repair, crew escape, and the US could cram two in the space a larger tank would have taken up in a freighter.
It was easy to operate at full effectiveness, with a relatively roomy turret (and a radio as standard). The most important (and controversial) Sherman decision was to keep it in service right until the end of the war. It was mass produced in vast quantities (and at the same time the US was supplying Britain with much of it's equipment and the SU with most of it's trucks). The Allies won with the Sherman's good qualities, and it's bad aspects were not enough to prevent success. It was good enough for the job. And I would argue that it was more than good enough for the job.
The T34? A fantastic tank when it was first put into service. It was badly let down by doctrine, but then so was any early war tank (that wasn't German).
Again, deliberate design choices meant that it was unreliable, cramped and hard to escape from. And famously most had no radio when it entered service. This reduced it's effectiveness. It was supplemented, but never entirely replaced, in SU service. And in the end, the Allies won. It was good enough.
It's features suited both the SU's situation, and method of combat. Unlike the US, the SU was in a desperate struggle for survival. Lesser design focus on crew survival and efficiency was understandable. Their tanks did not have to be shipped across an ocean. The Americans heavily supported the increased logistical chain an unreliable tank required. And Soviet tanks of whatever design did tend to be consumed more rapidly in vast battles of attrition, so a relatively low build quality was an effective choice, especially with an industrial base that was rapidly built up pre-war, and had to be shifted to avoid the German offensive. The T-34 suited the situation and choices the SU made. It was a good tank.
I would argue that although anyone could have made a better-constructed tank, the SU would have had to sacrifice more to do so. Their industry was not suited for high build quality. And SU doctrine would have lost thousands of tanks regardless of their design quality. They needed numbers to support their doctrine.
German tanks? The Panther in particular. They were used effectively. German doctrine compensated for the relative weak areas of their early tank designs. When faced with heavier-armored opponents, they could out-maneuver them. And their tanks were mobile to fit this doctrine.
A focus on quality suited a country that would be out-numbered and run short on fuel. They needed to maximise their combat effectiveness and minimise their logistical needs. And it matched (as the above examples also did, naturally), the nature of their industrial base, which was long-established and focused on quality.
However, their critical limitation was not the design, but the industry choices that limited their use. Assuming the war would be over quickly, not preparing for the logistics of supporting winter warfare in the SU, not mobilising their populace and industry early, and arguably not gearing itself for mass production effectively enough. These were choices that limited the production, and thus success, of German tanks during the critical years. But perhaps these were logical choices for a totalitarian regime that was reluctant to press it's people too hard. Overall I would argue that German tanks were amazing machines, and fit for purpose, but they didn't make as many as they should and could have during the critical years.
Which was the best? Personally I would vote for the T-34. It was not a fantastically-crafted or elegant machine. It had some really severe design compromises. But it got the job done. And it would have done even better with better doctrine (although perhaps this would also have exposed it's design compromises more).
But I accept that your own personal pick for best tank depends entirely on how you personally define "best".
