Argentina’s Nahuel D.L. 43 comes to mind. Argentina declared war on the Axis powers in 1945, and because the Nahuel was a decent tank and never saw action, I would certainly feel confident to face my historical enemies while driving around in one.
On a more serious note: an important question would be, precisely when those enemies would be faced. If the answer is “at any time during the war”, then the choice for a late model from either side seems like an obvious one. But if the answer is: “when the tank first saw battle”, then I’d be looking at the Soviet Union for innovative tank design. The T-34 would be my choice, because the Germans just couldn’t do much against it when it first appeared in 1941. It was this tank that forced Germany to reconsider its own tank design.
And yes, I know that by the end of the war, the T-34 had been surpassed by many other tanks.