The answer is really a moving target.
At the outset of WW2, the tank designs of all the combatant countries were essentially all shit.
I've seen it argued that 1940 era French tanks were the best in Europe. But they got steamrolled by other factors such as the fact that French manpower still hadn't recovered from WW1 (so they purposely designed tank turrets to be smaller at only 1-2 men, requiring the tank commander to double as gunner, instead of commanding the tank).
I also understand that doctrine played a role in early German tank victories, as the Germans had dedicated tank divisions that could take advantage of speed. While the French considered their tanks to be infantry support, so their mobility and organization was limited to walking speed.
The window before a version of a tank became obsolete during WW2 was very narrow. When the Nazis invaded the Soviet Union, they were surprised to encounter secret T-34 medium and KV-1 heavy tanks that were almost impenetrable to anything the Germans had.
But within a year, the heavy KV-1 more or less was removed from service since it was too slow and German anti-tank improved enough to make its armor redundant.
British tanks in general started off shit and remained so.
Italian tanks similarly were never respected.
The Czechs had some decent early war designs, but they were swallowed by the Nazis early with annexation and put into service for the Axis.
Japanese tanks were mostly shit.
The US started WW2 without a true tank. They had lots of time to fool around with prototypes due to their late entry. In the end, they mostly decided to forgo the light/medium/heavy tank doctrine of the day and mass produced the serviceable but all-round mediocre Sherman medium tank, much like the Russians did with the T-34.
The Sherman was nothing special in terms of firepower or armor, but it was decent enough and produced in such numbers that it won the day on the battlefield even if it wasn't the superior design.
The Germans Panzer Is and IIs which defeated Poland and France were nothing really that special, but won with numbers and tactics. The Panzer IIIs and IVs that fought the Soviets were better designs, but not any better than the Russian models they were fighting. The late war Panthers and Tigers were the best engineered tanks of the day. But were also heavy, unreliable, complex and expensive to make and in the end simply too few produced.
As
@DumbDude43 noted, the Germans also were producing mostly turretless StuG assault guns by the end of the war. They were taking the tank chassis from their old panzer designs and slapping on a casemate with a gun with very limited traverse in the thousands just for simplicity.
