Great topic, Mick!
I think the first movie I ever saw in a theater was the original "King Kong" in one of its Forties re-releases. An uncle took me. A family legend is that I asked him why Kong didn't have a dick. At least that's what they tell me...
Then there was "Annie Get Your Gun" (1950). That was memorable because I kept having to go the bathroom, the other patrons were annoyed, and my folks were embarrassed...
A standard feature of neighborhood movie houses in Brooklyn in the Fifties was that, the door would open on Saturday morning at 9 a.m., and kids would come pouring in to see three feature films, 25 cartoons, serials, Three Stooges shorts, etc. Parents loved it--kids wouldn't come home until five or six p.m. That's where I saw "The Conqueror" for the first time (1956). Unforgettable scene: Susan Hayward, playing outraged royalty just kidnapped by the greasy, yak-fat-dripping Mongols, asks The Duke how he intends to keep her in the style to which she's accustomed. The Duke's wearing an Oriental Bishop's Mitre and has his eyebrows tweezed upward, but he's still The Duke:
"By day, the pelts of my sheep shall keep thee warm. And by night, the heat of my body shall protect thee from the desert winds." I fell off the chair laughing.

Another impressive one: When "The Ten Commandments" finally made it to The Nabes, my local theater actually sold reserved seat tickets. Amazing! Still a great movie.