As OKW you know there is only one commander to choose.
As OST, well, lol.
Only one commander huh?
No desire to use the panther? The King Tiger with support? Smokes/Calldowns to get rid of the ISU supporting units? A lone Puma can dance on the ISU if you can force back other units.
OST has no counters to the ISU? Panther, PZ4 Spam, luring into Pak 40s, mines, any wide variety of call downs that allow you to suppress/force back the ISU support and isolate it?
Jagdtiger is by NO MEANS necessary to win against an ISU. I'll cede that on some maps, it may be the best/easiest choice. But it is never the only choice.