There are, as far as I can see, four cultural landscapes in Europe featuring river valleys, and I'd say the most famous is the Loire Valley (the others are the Rhine Valley in Germany, the Danube Valley in Austria, and the Douro Valley in Portugal). An incredible mixture of history, architecture, and culture, it is worth visiting for any number of reasons, but the chateaux certainly beat all the others. They are all masterpieces of Renaissance (and partly Baroque) architecture, especially the famous castles of Chenonceaux and Chambord (the most beautiful of them, with the famous double staircase by da Vinci). Together with great historic cities such as Angers, Tours, Blois, and especially Orléans, the Loire Valley captures the essence of French history and culture and is definitely worth a visit (more visits really).