Psycho and not because it's horror rather it's because it's one of the few movies I know from those early black and white times where the acting doesn't seem completely exaggerated and unrealistic.
I feel like a lot of movies early on were in times where most people making them couldn't figure out how to make things immersive and the acting usually seems a bit too exaggerated as if the emotions are being forced out and they don't seem at all natural. I feel like a lot of older movies treat the audience as if they are too stupid to understand what emotions the characters are trying to project so everything is a bit overdone to drill it into them.
The few movies that respected the intelligence of the audience and allow for subtle and realistic emotion are the ones I most appreciate. Also I love those movies where they surprise the actor with things they didn't except to get a realistic reaction. Those tend to be the movies that most people will remember but just like today there is a lot of shit acting and it's difficult to immerse yourself into a story.