IS2 armour is quite superior to Tiger and feels like more than KT, might be definitely wrong on that fact. The fact is it is more difficult to Penetrate IS2 since Panthers ricochet it most of the time, being its only best axis AT option in game. It feels absolutely annoying when it ricochets many times and you cant get around because of how mobile and deadly it is.
What makes IS2 great simply, is armour, AOE, against infantry is better than KT and T, most of all it is the speed that kills. Very difficult to counter it is indeed.
The only thing that makes IS-2 good is its armor.
Its AT DPS is bad, but at least penetration chance is reliable, its AI is, exactly like KT, a slot machine of hit and miss(mostly miss).
And no, panther is not best axis AT option, its best STOCK AT option, there are at least 3 doctrinal options that have 100% chance to penetrate IS-2, KT also does not struggle in any way against IS.
Tiger have LONGER RANGE, higher penetration(stat wise its 10 lower, but all of allied vehicles, short of IS-2 and arguably comet when put on the same shelf as panther have lower armor then their axis counterpart, so at the end it just has easier time against all the allied armor then reverse), much better scatter and rate of fire, access to better survival tools depending on doctrine(however plain old blitz is also vastly superior to capping a fucking point).
IS-2 will beat Tiger 1v1, but Tiger is overall much stronger and better tank that actually inflicts massive damage instead of just taking it.