When was the soviet union most powerful?

My math tutor (who lived in the soviet union) realized its down fall with a single billboard. he used to teach at a university. while in st. petersburg, he saw a sign in the late 60s to early 70s that said something along the lines of "Bus drivers! unhappy? come work in moscow! you will be guaranteed the same (wage) as that of teachers in universities!" he said that fixing everybody's wages as equal was what really caused the fall of the soviet union. when asked when it was most powerful, he said under Stalin, as it was under his dictatorship when a professor made 500 times as much as a custodian (for example). what do you think?