the.com/holy grail

means The ultimate, hardest-to-attain goal in a given pursuit — the one prize that would make everything else complete.

from From medieval legend: the cup supposedly used at the Last Supper (and to catch Christ's blood at the crucifixion), which Arthurian knights like Galahad and Percival spent their lives questing after. 'Grail' itself comes from Old French 'graal,' probably from Medieval Latin 'gradalis,' a wide serving dish — so the word started out meaning something as humble as a platter before legend turned it into the most sought-after object in Christendom. The 'holy' got stapled on to mark it as the one true, sacred grail among ordinary dishes. By the 20th century the phrase had drifted fully into secular use, for any coveted, maybe-unreachable goal — science has its holy grails, so does sports, so does everyone's inbox.