Mona Lisa
The Mona Lisa, also known as The Gioconda, has gained the status of “the most famous painting in the world” due to a combination of various bohemian predilections and series of events, most of which evolved and took place during the 20th century. “The most famous” is not necessarily the “most beautiful,” as this extensive Wikipedia article appears to imply — but it undoubtedly is beautiful, and will remain such even if it loses its celebrity status.