The most famous painting in the world is a portrait. Her face is imprinted on everyone's mind and this one having suffered a recent offence, we are inspired this week by faces, their expressions, their universality, their uniqueness. Our artists like to explore all that faces have to offer, here is a selection of works that, in their own way, are all modern Mona Lisa. Behind each portrait, a whole mystery is hidden: who are they? And who are we?