The Mona Lisa by da Vinci at the Louvre is not the biggest painting, but is perhaps the most famous. I am always amazed by the crowds surrounding this piece and snapping her picture like paparazzi.
I completed this sketch very quickly while my tour group was looking at the painting directly across from Mona. It is not the greatest drawing I have ever created, but for me, it captures the feeling of my experience. (I think Tintin is in it too.)