What's the best (or worst) character unmasking of all time?

What's the best (or worst) character unmasking of all time?S

So we were talking about our issues with "John Harrison" turning out to be Khan in Star Trek Into Darkness. But what's the absolute best moment where a character is unmasked in science fiction and fantasy? Or the worst?

Please include a clip or image of this huge character reveal. And mention the name of the show/movie/book, and exactly why it's so great. Thanks!

Top image: Doctor Who, "Utopia"

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