The Mona Lisa. I'm glad that they've placed it in an obscure location, forcing people to walk through various halls first so that visitors inadvertently sees other great works, even if they don't recognise their significance. It's funny watching everyone crowd around taking photos of it instead of taking it in through their own eyes.
It would be great to have a week to spend just exploring the Louvre! The building itself is an expansive work of art.