Saving Private Ryan As noted, the landing on Omaha is gut wrenchingly realistic, and powerful.
Alien. Sorry, but the Alien busting out of McDowell's chest trumps even the Exorcist - barely - as the scariest moment in cinematic history.
Jaws. When Brody sees the shark for the first time, noting laconically, "You're gonna need a bigger boat."
Full Metal Jacket. Lee Ermey's GySgt Hartmann is the perfect rendition of a hardass Marine DI, and every line he speaks is unadulterated, brutal, and gloriously poetic.
Sorry, John Hurt.
My older brother never served (multiple knee operations in childhood) and I've told him that if he wanted a real taste of life as an Army (or Marine) recruit that he should watch Full Metal Jacket.
However,the only physical violence I ever received from my DIs was when the junior DI (E-5,just back from 'Nam) took me behind the barracks because he,a southern boy,didn't like us "Yankees".
One of the funniest moments was when Hurt reprised that scene in Spaceballs.
Miller's Crossing, where Albert Finney is in the house during the shootout, with "Danny Boy" playing in the background...
"Jaws. When Brody sees the shark for the first time, noting laconically, "You're gonna need a bigger boat"
The scene I can't get out of my mind is at the beginning of the movie, when the girl is out swimming in the dark and the shark pulls her under the water and she bobs up with that startled look on her face and then she disappears under the water.