Hmmm tough choices. Tiger or Panther WWII. The panther hits harder due to its higher muzzle velocity but what people don't often know is that is only out to 1,200 meters beyond that the weight of shell put's the 88 above the panther in armored piercing capability.
Ah never mind I choose the Stug IIIG!
Modern MBT is Abrams or leopard II.
And yes lend lease did save the soviets.