I think in this case, "ultime" probably means "final". Because if the Louvre is trying to say that Saint Anne is really de Vinci's ultimate chef-d'oevre, than they are deluding themselves. Or maybe no one really likes The Mona Lisa. It's not like it has its own wall at the Louvre or anything.
Also, can we please talk about the frenchification of non-French proper nouns? Léonard de Vinci? Really? They also like to talk about Michel-Ange and La Caravage (at least they make up for implying that his given Italian name wasn't good enough by giving him his own pronoun though).