This is partially inspired by @UsedJavelin's thread about Seven Samurai.
I watched a lot of the Western "film canon" when I was younger, and lately I've just been craving some good "artistic" movie content. But I don't trust the :reddit-logo: crowd nor the letterboxd nerds for serious recommendations, so I turn the question to my comrades
#1 has to be Z (1969) by Costa-Gavras, about the fascist coup in Greece in the 60s. Comrade made, you can just feel the anger of all the people involved. People talk about old movies feeling slow, this movie fucking pops. You can watch the whole thing for free in good quality here, honestly if you're not all in after the first 5 minutes, or really after the title card that says "any similarity to real person or events is not coincidental; it is intentional" followed by the speech at the fash gathering in a smoke filled room, then you won't like it.
If you like that, his State of Siege (1972) is also a classic, about when the Tupamaro urban guerrillas in Uruguay abducted US torture specialist Dan Mitrione. Just total :based-department: stuff. Filmed in Allende's Chile.
I'll second the people who mentioned Casablanca and The Bicycle Thief.
Bridge Over the River Kwai (1957) and Wages of Fear (1953) are also cool.