I'm so glad this thread exists! Yay!!
El Lissitzky's
The Constructor:
(I had read about it in a book on Constructivism and I don't have the exact excerpt about this piece but I remembered that I rather liked it; it said that the photomontage is a self-portrait of Lizzitzky with a perfect circle, demonstrating the power of the human mind...or something to that extent?)
wikipedia