I like Virgin and Child in an Interior by Jacques Duret c. 1435.
To me, it seems one of the more down-to-earth paintings there is of the Virgin Mary and baby Jesus.
I love the expressions on Their faces and I think it illustrates perfectly the tender vulnerability of the mother-child relationship.
As a mother, I feel vulnerable to all the worries that go with being a mother, and knowing there's always a chance I could lose my child to disease, accident or violence, and that I'm not always going to be there to protect her from this crazy world that I know is only getting crazier.
The baby is vulnerable as well, totally helpless and dependant on protection.
The painting just sums it all up. (Plus I like the composition and lighting of the work as well.)