The Ark of the Covenant
We will start with a big one. Besides the Holy Grail, no other religious artifact has been the subject of as many worldwide searches as the Ark of the Covenant. According to Biblical legend, this ancient chest houses the tablets with the engraved Ten Commandments that Moses presented to the Hebrew people.
Legend says that the Ark was kept at the Temple of Solomon, but all traces of the Ark have been lost since the temple was sacked by King Nebuchandnezzar II and the Babylonians in 587 BC.