It would be difficult to pick one from the various genres, considering whether it would be inspirational, frightening, awe, or setting the tone for the entire film... For the latter, I would go all the way back to Sam Peckinpaw's The Wild Bunch with the opening scene of the children casually torturing a scorpion in a red an't nest.... the casual violence is nearly unending following that opener... But a close second would be the introduction of the monolith in the opener for Stanley Kubrik's 2001 A Space Odyssey... especially with the masterful accompanying music of Richard Strauss' "Also Sprach Zarathustra....