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Hard to compete with the mcguffinness of the Maltese Falcon (with Spade presenting it in-film as the ultimate mcgufffin), but still, I'm partial to books. Books are magnificent mcguffins. I love any lovecraftian story revolving around the necronomicon, I love The Evil Dead for it, and I for wish more movies à la 9th Gate. Books are under-used as mcguffins.
Failing that, microfilms. Cleverly concealed microfilms. The world balance depending on a microfilm. Perfect for Cold War tension, intensity, mystery and triple-crossing (can't really beat that setting), with the delightful contrast between the macro and the micro. Geopolitical stakes on a time stamp, sometimes quite literally.
The digital age deprived movies of microfilms (and phone booths). How will cinema survive this ? Badly, I say.