You said Panthers, so I assume you mean more than one. Lets say 2 Panthers. 2 Panthers are dedicated Anti-Tank, and cost almost as much as an IS-2, which is dual Anti-Tank / Anti-Infantry. Even so, the IS-2's gun is as effective as a Panther on Tanks, and a Sturmpanzer on Infantry, the trade-off is the slow rate-of-fire. I would hope that 2 dedicated Anti-Tank's can kill one breakthrough Tank.
Alternatively, yes, a single Panther can kill a single IS-2 from the front, but that's just RNG. A T-34/76 can beat a Panzer IV from the front as well, and is even more likely to have success with this than a Panther vs IS-2 is.
Actually with a Panther you can micro in such a way that the IS2 is constantly stuck with Turret traverse as your drive around it getting hit after hit in as it helplessly lumbers backwards to try and get away from the Panther thats on it like a bee hive.
This will net you a kill with a Panther vs an IS2 1v1. Otherwise I dont know a Panther and a single faust hit? A Panther and a single Shrek hit. A panther and a single Pak hit. A panther and a Stug. A panther and a Brummbar. A Panther and a P4 a Panther and an Ostruppen squad and and and....
Also a single Tiger thats cheaper..
IS2 is hardly the beast its made out to be. Now if it had 1080 HPs rather than 960 then you may have a point.
As it stands it takes far too much of a beating from a lone Panther to be even remotely considered OP.