The absolute very best of something. Typically, an adjective that refers to something that is the best it can possibly be based on qualities of being exceptional or nothing but the best.
My professor is the crown jewel of the English department; she's the very best and the most knowledgeable.
The Vespa Granturismo Vintage is the crown jewel of scooters.
Jordans are the crown jewel of the sneaker world.