I don't even think the IS-3 saw actual service in the WWII, to many mechanical problems. Though otherwise Russian tanks were the least prone to mechanical failure.

The Panther btw is not a heavy tank, the Germans themselves classified them as medium tanks and deployed them as such. This tank gets my vote for best WWII tank. It could kill a T34-85 at a range of 2 kilometres, had descent mobility and excellent armour.
T34-85 could destroy a Panther when close enough, but all things considered I think it's outclassed by the Panther. The only other two tanks I can remember that have a fighting chance are the M26 Pershing and the IS-2, wich both fall into the heavy tank catagory.

Costwise though, the Panzer IV w75 is better.