The Ark Of The Covenant
As seen in: Raiders Of The Lost Ark
The Power: The Ark Of The Covenant is described in the Bible as the sacred container in which The Ten Commandments rested - "and thou shalt put into the Ark the testament I give thee" (Exodus 25:16). The source of God's presence on Earth, it also acts as a kind of divine flamethrower. According to Raiders' Marcus Brody, "an army that carries the Ark before it is invincible."
The Real Life Version: The holiest object in the Bible, the Ark is believed to be a gold plated wooden box, (4ft x 2ft by 2,5ft) and, because it is too holy to be put on the ground, or touched by any one but the chosen, it is carried on poles inserted into rings, famously circling the walls of Jericho behind the trumpets to begin the walls tumbling down.
Raiders has the Well Of Souls, the snake filled chamber that is the resting place of the Ark, in Tanis, Egypt but the real well of souls is actually the name of a natural cave beneath the Dome Of The Rock on the Temple Mount in Jerusalem. Certain legends suggest the Ark was hidden there but removed when the Neo Babylonians destroyed Solomon's temple. Guesses for the current whereabouts of the Ark include Ethopia, Zimbabwe, the Middle East and even dear old Blighty - historical boffin Graham Phillips suggests the Ark was taken to Mt Sinai by the Maccabbees (the Jewish liberation group, not the indie band) where it was gathered up by Ralph de Sudeley, leader of the Templars, and taken to his estate in Warwickshire.
The Movie Version: It wasn't George Lucas who came up with the idea of Indy chasing the Ark. The notion came from fellow San Franciscan filmmaker Phillip Kaufman who was told the story of the Ark Of The Covenant as a child by his dentist. There are also elements of truth in the Nazis interest in archaeological artefacts. Hitler was obsessed with finding religious icons to legitimize himself as a Messiah type figure and there are numerous stories of Nazis hunting down treasures in all corners of the globe.
Effectiveness as a McGuffin: 5/5 The perfect movie Macguffin. Rooted in credible folklore (but not hamstrung by it) and full of mystery, wonder and danger.