In no particular order:
Alien: Jones the cat recoils and hisses at the full-grown alien, which is standing up (unbeknownst to him) behind Harry Dean Stanton. The audience knows right then and there, based on the angle of the cat’s stare, that The Game Has Suddenly Changed.
The Caine Mutiny: The moment of silence, punctuated only by the clacking of the steel balls, just after Capt. Queeg loses it on the stand. You just know that it’s all over, and Keefe and Marek aren’t going to be convicted.
Se7en: Morgan Freeman opens the box. Right away, before anyone says a word, you know what’s in it.
Casablanca: Renault’s look just before he decides not to tell the police that Rick shot Maj. Strasser. Faced with the necessity of making a moral decision for the first time in a while, he slam-dunks it.
Dead Again: Mike looks in the mirror in his dream and realizes he is Margaret, not Roman. So obvious, but it was handled pretty deftly, and the first time you see it, it works perfectly.
And five more for the hell of it:
Vertigo: When Judy goes over the edge of the tower at the end. That look on Jimmy Stewart’s face . . .
Bull Durham: Bat boy: “Get a hit, Crash.” Crash: “Shut up!” I just about wet my pants the first time I heard that, and it hasn’t gotten any less funny.
Close Encounters of the Third Kind: Dreyfuss is on the phone talking to his wife, the model is taking up his entire living room, and Devil’s Tower appears on the news report, and you’re thinking, “Turn around, dammit!”
Bambi: When Bambi turns around and says, “We made it, mother! Mother . . .?” Cripes, that STILL makes me cry.
The Godfather: Moe Greene gets shot in the eye. Nothing crucial to the plot or paticularly touching, but it was pretty frigging cool.