Paintings aren't like sports. There's no GOAT. And even in sports, it's debatable. That painting is famous, largely, because it is one of the first naturalistic portraits in history. And of the those, the most beautiful. It was painted by Da Vinci, after all, and he took painting to a new level, and he changed portraiture with this painting. Also, for the enigmatic smile, and for the history of it, including being stolen, and recovered. The Italian Renaissance created for us in the West, by a huge measure, what we understand as beauty. Rightly or wrongly. But between history, reality and lore, it is considered the most valuable painting in the world. That doesn't make it the greatest. But if we made a Top-20 of important ones, or even likely Top 5, it would be there.