Alfred Hitchcock said the McGuffin is the thing that all the characters want but the audience doesn't care about. The secret plans, the microfiche, the love letters, the hostage, the vial of airborn flesh-eating disease, all that shit. He might have spelled it MacGuffin. I don't know.
What is your favorite McGuffin?
Is it the Maltese Falcon? The data inside Johnny Mnemonic's head? The briefcase with the glowing interior? Inquiring minds want to know!