This historic library is the jewel in the crown of our city.
jewel in the crown
idiomThe most valuable or important thing or person among others of its kind.
Origin
The phrase historically referred to India under the British Raj (1858–1947), the rule of the British Crown on the Indian subcontinent.
The company considers its new research facility to be the jewel in the crown of its global operations.
While the museum houses many impressive artifacts, the rare diamond necklace remains the undisputed jewel in the crown of the entire collection.