Operationally speaking the Sherman was the best tank because it featured the best all round reliability. When the T-34 started out they were losing 60 out of every 100 tanks for every 100kms driven on marches alone. The German Pz III did much better than this marching over the same ground and distance losing about 1/3 tanks, so in the early war period the Pz-III was the best.
As the war progressed the German ammo manufacturing failed to keep pace and the replacement ballooned into the Panther monster tank, which had appalling reliability.
In the late war period Russian tank loses [read T-34] were around 13% for every 100km driven. But this was more of a reflection of the improved emphasis the Soviets placed on parts and the logistics tail of their combat units. Meanwhile Sherman reliability topped all in no small part to design.
When you look at the Tactical side, the most important quality is fire power as this is the mechanism that kills and that's what settles the battle. In terms of firepower the most important criteria is ROF or rate of fire. Obviously nothing can be stated as simplistic as that, because there's a point where ROF can't make up for poor ammo penetration. This is why the failure of the German ammunition concept -when the AP 40 shots disappeared - became one of their biggest failures.
In the initial stages the Pz-III was the best tactically due to its high rate of fire [2:1 against the French/ Brits and 3-4:1 against the Russians]. By mid war the Panthers and Tigers featured lower ROFs while allies remained the same [8 rpm Germans Vs 3-4 rpm T-34s and 10-12 rpm for the Shermans]. So the heavy armor and fire power worked against the Russians- to wars end, but didn't work so well against the western allies.
It’s hard to show just how important ROF is but if you take 2 tanks that are equal in all respects but blue out shoots red by 3:1 then- in an even battle -blue will inflict 10:1 kill ratios. Even outnumbered, this kill ratio can be maintained due to the smoke and fog of war. This allows better trained troops to section off large numbers into mini firefights near one on one. The weak point of the ROF argument is that it does require 'well trained troops' and to preserve those you do need good armor over enough of the front quadrant to allow the said 'well trained troops' to survive.
So I guess the bottom line is you can't pin down the best tank unless you have a proper strategy and tactics. If you’re forced to change mid stride, like the Germans you have to adjust your designs accordingly. Even the Poorly designed T-34 is the best tank at the end of the war cause the Soviets finally got there tank; logistics and strategic situation right.