The German produced by far the most powerful and deadly tanks of WWII.
Take the heavy Tiger Tank. It weighed over 56 tons, it was equipped with an 88 mm cannon and 92 rounds of ammunition. Its frontal armor was 100 mm thick and no allied tank could knock it out even at close range, at least from the front. The Tiger, conversely, could destroy Allied tanks over 2,000 yards away.
The even heavier Tiger II or King Tiger which was used during the Battle of the Bulge weighed in at nearly 65 tons had even thicker armor making it virtually an indestructible fortress unto itself on the battlefield.
The problem was two-fold with these types of tanks: Their sheer size and complex engineering made them impossible to mass produce unlike the Soviet T-34s and American Shermans which were massed produced by the tens of thousands. In addition these heavy tanks were terrible gas guzzlers and Germany was chronically short of fuel especially towards the end of WWII.
I saw a program on tanks on the Military Channel about tanks and many experts regard the medium sized German Panther Tank as the superior tank of WWII. Although smaller than the Tiger, it was faster and more fuel efficient than the Tigers and it also had sloping armor.
The two best tanks in operation today are the American M-1 Abrams Tank and the German Leopard Tank.
The increasing size of tanks also limited their movement,as many bridges would not support their weight.
I think I'd rank the Brit Challenger II ahead of the latest Leopard variant.