I like this one, "As it was, as it is, as it will be", a Soviet poster from 1922:

It really succinctly captures the progressive historical project of communism, that even that early in the USSR's life they were absolutely clear in their plan to advance science, industry and the quality of life of the workers and peasants and confident that they could achieve it.

I also really like these postcards from 1959, showing traditional dress from various SSRs:

Edit: The pictures came out a different orientation than in the preview! On the top left are Belarussian, Armenian, Georgian and Azerbaijani peoples, on the bottom right are Turkmen, Tajik, Ukrainian and Uzbek peoples. I love how it shows the celebration of the various nations of the USSR and their differences, in such a commonplace way (a set of postcards) that it's clear it was not some kind of controversial or artificial statement.

But my absolute favorite is this one:

"Through Worlds and Centuries"