As per this rule stated in this lesson - "de la / du /des / de l' ('of the') always follows the superlative, even when in English we might say 'in'."
But,
- Ce sont les traditions les moins connues en France. [These are the least well-known traditions in France.]
How come 'de' does not fit here, because the rule says it should ALWAYS be de???
But its also,
- Paris est la ville la plus visitée de France. [Paris is the most visited town in France.]