The more I think about this topic, the more I just keep going back to Sergio Leone. So many great scenes.
--The opening credits and initial gunfight in Once Upon A Time In The West. The gunmen lazily waiting for the train followed by the slow, tense conversation with Bronson's Man With The Harmonica (culminating with one of the all-time greatest dialogue exchanges: "Looks like we're shy one horse." "You brought two too many.") perfectly offsets the incredible speed and violence of the eventual shootout. The "natural" soundtrack before and after the gunplay is the icing on the cake.
--And speaking of great soundtrack usage, one of my favorite moments in Once Upon A Time In America comes when DeNiro's Noodles visits the crypt of his dead friend, accompanied on the soundtrack by the haunting pan flute song his dead friend used to play. The music, it turns out, is not just for the audience's benefit -- it is being piped into the crypt, a fact which only becomes apparent when the door closes, which shuts the music off. Realizing this, Noodles opens the door again and the music returns.
--Of course, no praise of Once Upon A Time In America would be complete without mention of Burt Young's fantastic "cock insurance" scene, a rare example of Leone allowing such a long stretch of dialogue.
Seriously, I could list almost every scene from America and it would qualify: The boy gingerly eating his way through the pastry he was going to use as payment for sex, the penis lineup, the baby switching scene scored to "The Thieving Magpie", Noodles returning from his days-long opium bender to find Max literally sitting on a throne, and on and on and on.
Heck, I could just list scene after scene of rugged, leather-skinned badasses just staring at each other, like this one from For A Few Dollars More.
And I didn't even mention The Good, The Bad and the Ugly...