My choice for the greatest MacGuffin in cinema history is the One Ring from The Lord of the Rings trilogy. That would arguably make it the greatest MacGuffin in literary history too, since it's based on J.R.R. Tolkien's novels, but I'd give the literary crown to the Holy Grail, which so many writers throughout history have used to drive a story forward without the need to explain why it's needed -- "It's the Holy Grail, man, who cares 'Why?' Let the quest begin!" Plus, virtually every MacGuffin serves as a pseudo-Grail anyway.
Like that mythic cup, the description of what it actually does is often left vague (maybe it's eternal youth like in Indiana Jones and the Last Crusade, maybe it's the answer to some conspiratorial mystery like in The Da Vinci Code). It's primary purpose is to be found or defended at all cost. It's actual practical functionality is secondary.
The Lord of the Rings has a near-perfect mythic narrative structure. What exactly the One Ring does is never fully explained, nor does it matter. We know from The Hobbit and later chronological instances that it makes the wearer invisible. That alone doesn't seem enough to make it so sought after and the crux of the major conflicts to come. We learn that it was forged by Sauron in the heart of Mount Doom, and its power is enormous. What exactly is that power? We're told that other power rings exist, and the bearer of the One Ring can control them all. Bottomline, it's a MacGuffin.
The actual power is moot, the point is that the bad guy wants it, the world is at risk if he obtains it, and the good guys need to destroy it. The One Ring is especially cool, because possessing it causes the eventual corruption of the user, and we see it tempt and taint characters who come into contact with it -- Gollum, Bilbo, Frodo. Again the explanations for that corruption are left vague, we just see enough to move the story along, the rest is left to our imagination and suspension of disbelief. The fine point details are unnecessary.
Movie history is loaded with excellent MacGuffins -- the ruby slippers in The Wizard of Oz, the jewel-encrusted statuette in The Maltese Falcon, the briefcase in Pulp Fiction, the Death Star plans in Star Wars, the meaning of "Rosebud" in Citizen Kane, the Tesseract in Marvel's The Avengers, the ark of the covenant in Raiders of the Lost Ark, the Genesis Device in Star Trek II: The Wrath of Khan, the soldier in Saving Private Ryan -- but the One Ring rules them all!