Veronese, The Family of Darius before Alexander, 1567. Oil on canvas, 93" x 187."
I stood in front of this Veronese at the National Gallery for a long time puzzling over over how the composition could work so well when it felt like so much of it was crammed over to the right. Then I saw it: the two interlocking waves of value, like Yin and Yang, light on top, dark on the bottom.
Some of the best pictorial ideas sound trivial when described in words, but are phenomenal in practice. This is one of my favorite things about pictures.