Which of them exactly?
Because ZiS is worst ATG in game in terms of actual AT kill times(assuming there is muni to feed USF one), SU-76 is excelling to the point where no one ever gets it.
ISU excels exclusively in AI and its AT DPM is lower even then that of SU-76.
Guards hardly excel at anything, they are just supplemental in both roles.
T-70 scouting/sight providing role was butchered long time ago.
ZiS has unchanged AT power since release and barrage effectiveness went down significantly.
Then play soviets.
OK, lets play with your cards:
ZIS can keep at bay and Pen any tank, even at long range + the barrage + 6men. As an AT gun, is more than ok (inferior than its axis counterparts, but even good).
SU76 has more range and Pen than the StugG + barrage + almost same cost as stug.
ISU is a very long range Brummbar + can pen medium tanks and other TD.
T70 is a wiping machine since day 1 + scout + good at hunting halftracks.
And I not even mention the T34/85 (its a OKW P4 on steroids).