Which artworks are the most sacred?

Today I read a paper in which people were asked to identify one or two artworks that they consider sacred - not in the sense of being about religion, necessarily, but in the sense that some people think of their country's flag as a sacred object. They did this in two different studies, and the same four artworks were most commonly mentioned in each study. Want to guess what they are? I'll put the answer in the comments. Hint #1: They were all paintings. Hint #2: The study was about collective beliefs and values, so the question is largely about having a good idea of what other people think.