For me, The Watchmen has one of the most well thought out openings ever.
Minor The Watchmen spoilers below.
It has enough twists on history to get me into this world, from The Comedian killing JFK to the MP's firing on the peaceful protesters. Just enough character scenes for me to not need some elaborate introduction into their dysfunctions and insecurities. A complete juxtaposition in so many clips. As well as being put to the fitting track of Bob Dylan's The Times They Are A Changin'.
So /r/movies, what's your favourite opening scene and why.