I would say that the IS-2 was better than the Panther in a one-shot situation. Better frontal armour, smaller size (though not much) and a much heavier gun (though not much better for the AP part). And that tank was definately on the scene prior to the M-48.

Also the 88mmL71 was definately superior to the 75mmL70 of the Panther. The L51 of the Tiger I was not much better, and in some cases it was slightly less effective than the 75mm of the Panther.