What's the greatest MacGuffin in science fiction or fantasy?

As coined by Hitchcock, a MacGuffin is an object that drives the plot of a story forward — even if it's intrinsically meaningless. Often, it's the object everybody wants to get their hands on. Like a treasure map. Or a tesseract. But what's the greatest MacGuffin in all of science fiction or fantasy?

Please include a picture, clip or book cover that includes your MacGuffin if possible. And please mention what it is, where it comes from, and why it's such a great object. Thanks!

Top image: The Tesseract from The Avengers and Captain America.