German would have to be the Panther. By the time of the G model most of the teething problems had been corrected. Extremely lethal tank killer. While the Tiger was a sledgehammer breakthrough weapon, the Panther with it's high velocity gun was like a scalpel. First shot, first kill ratio was higher than any other German tank. Also when you kill on first shot, you move, giving it better survival rates. Much cheaper to produce than the Tiger series, an easier tank to learn on as well, It influenced tank design for years after the war. In the west I know from talking to a few vets and in literature that a lot of time when units reported they were under fire from Tigers it was actually Panthers that were chewing them up.
As has been said time and time again most experts agree it was almost the perfect balance between speed, armour, and firepower.
As a side note since it so influenced Panther design the T34/76. It was a revolutionary design no one else had come close to in the period of it's release. It too did have teething problems but by late 42 was good to go. Just an all round efficient tank. The biggest benefit from my view is it was built for war in Russia with few roads and horrendous winter conditions. It's ground pressure weight let it move over all sorts of terrain that German tanks were incapable of even attempting. Sloped armour gave it increased protection while not increasing weight, the 76 gun was a good gun but the optics weren't great. The motors were bullet proof although the initial transmissions were not the greatest. General overall maintenance was very simple for even raw crews. With proper radios, command and control, and proper inter-arms coordination the Russians could have ended it sooner before the Germans stepped up their armour I have no doubt...
