Though seemingly obvious, one of my choices would be the original Equus poster design by Gilbert Lesser.
The simple, black and white, puzzle-piece arrangement is now iconic. The straight line of the neck, accentuated by the repetitive perpendicular rectangles that make up the mane of the horse, runs into the awkward presentation of the top view of the horse's head. This allows both of the eyes of to see us.
We are also unsettled by the mouth which is contorted with anger...Pain?
This is the puzzle box Equus. The mind play.
Contrast this to the poster design for the current West End stage incarnation with Daniel Radcliffe.
The black and white theme is given a nod, but the puzzle becomes less a geometric jigsaw; not only more human, but, (quite Madison Ave like,) more sexual.
The centaur imagery which permeates the play is flipped around.
But the poster still manages to capture the mystery.