I was very fond of my Spirograph set as a kid so naturally always liked this famous spiral pattern (designed by experimental filmmaker John Whitney) and the silhouetted figures (drawn by longtime Bass associate Art Goodman) put together by Saul Bass (1920-1996).
This one being the most recognisable of the four created:-
Out of all the posters, and there are quite a number of them, the strangest has got to be this one from1961 (Poland) by Roman Cieslewicz which is just about saved by the spiral pattern of the fingerprints and the target on the skull:-