The Death Star Plans are still the best MacGuffin in any film

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The ark was a deciding factor.

The army that carried the ark before it would be invincible.

The Ring is even more relevant to the plot, theme, and characters in Lord of the Rings. It affects them a lot more than the Death Star Plans.

I might also argue Rosebud. Rosebud was used to deliver an emotional climax as much as a physical one, and to the audience instead of within the movie. When it's shown what Rosebud is, the weight of Citizen Kane's story - it's whole point - basically just reaches out of the screen and stabs you in the feels. Because of this transcendent quality, I'd rate Rosebud pretty highly, too.

It's effective, but the Best? I dunno.

I like the more subtle ones like in Wizard of Oz (Home), or Angel Heart (Johnny Favorite)

I like when he says "And help take a bite out of crime". Classic MacGuffin!

If "it was deciding factor and used againist the bad guys", then is not a MacGuffin

Agreed!