To me, T2 -> T1 for a flak or puma is best against Soviet T1.
It's safer to go T1 -> T3 for a PII most of the time. And get a couple of puppchens.
Rushing for a KT would only work against a Soviet aiming for dual IS-2s/ISUs. Most of the time a panther is the best choice.
This^. The best way to slow down IS2 or ISU is to harass their fuel constantly. Remember you don't have to cap, just decap to stop their gain of resources.