IS-2 is not the greatest tank of WWII.
The Super Pershing had the best gun of WWII and the Tiger II had the thickest armor of WWII.
The Super Pershing, however, only had a marginally better gun than the TII and yet had substantially inferior armor. The Tiger II had much better armor than either the Super Pershing (of which 1 total was employed in WWII) or the IS-2. Granted, it had a pretty obscene cost and it was prone to breakdowns, but in a combat engagement I'd rather be sitting in a Tiger II.
The IS-2 had limitations of its own, namely that its gun didn't fire a very high velocity projectile and that its penetration wasn't very good. While it would have been able to penetrate a Panther or a Tiger I with ease, it would be incapable of penetrating the frontal plate of a Tiger II at any range with any cartridge, and that's obviously a significant advantage for a Tiger II firing a high velocity 88mm AP cartridge. The IS-2 also had another issue, namely that its cartridges were separated into two pieces, with the powder charge separate from the shell.
"German Army data on the penetration ranges of the 122 mm A-19 gun against the Panther tank showed it to be much less effective than the Soviets thought:
the A-19 gun was unable to penetrate the glacis plate of the Panther at any distance, and could only penetrate the bottom front plate of the hull at 100 m.
[3] It was however the large HE shell the gun fired which was its main asset, proving highly useful and destructive in the anti-personnel role. The size of its gun continued to plague the IS-2,
the two-piece ammunition was difficult to manhandle and very slow to reload (the rate of fire was only about two rounds per minute). Another limitation imposed by the size of its ammunition was the payload: a mere 28 rounds could to be carried inside the tank."
In other words, the IS-2 was an extremely effective heavy tank in WWII that fulfilled its role with exceptional armor penetration and armor protection combined with unusually good mobility, but suffered from enough issues to dock its performance overall and lacked the adequate means to compete with the greatest tanks of the era (although well equipped to obliterate everything else on the battlefield).