I feel like everyone has been told at some point that Citizen Kane is the most classic, best film of all time. And the plot is pretty ok, but what's really great about it is the cinematography. But first, look at this shot of Kane Wolf-of-Wall Streeting a bunch of 30s newspaper exes.
As for its cinematography, one of the cool techniques they used a lot was having stark silhouettes against starkly lit background.
They also did a lot of cross-fading shots, it felt like they were just starting to get a grip on using montages to show lapsed time.
One other technique they used, which I hadn't seen in other films yet, was focusing on objects in scenes to get a point across, instead of the actors explaining what's going on.