Grail Watch
From Chronopedia
The Grail Watch: A Definition
It is the ultimate gap that needs filling. Unobtanium, Rocking-horse poo, and every other missing element in the solar system combined. The one watch you need - the one you crave more than any other. The one that will complete your collection, complete your life. It is the one you will sell a kidney or a first-born for, the one that makes you weak at the knees just gazing at photos of it. It's the one that makes you green with envy when another collector posts about adding it to their collection. The one you lose sleep about, the one you dream about.
It is...THE HOLY GRAIL
(Luckily it's not the same for each collector so we all have a chance, however long the odds).