A few of my favourite classics. In chronogical order:

Metropolis (1927). Fritz Lang's silent film about an exploited working class. It's basically the birth of serious sci-fi filmmaking. I'm not sure I'd call the politics explicitly leftist in a modern context, but it's certainly not right-wing.

Forbidden Planet (1956). A sci-fi adaptation of Shakespeare's "The Tempest". This movie heavily influenced the original Star Trek TV series, to the point where it's obvious that Gene Roddenberry was straight-up copying concepts from it. Leslie Nielsen (yes, that Leslie Nielsen) plays an extremely serious James-Kirk-esque starship captain ordered to investigate the crash of a science vessel on a remote uninhabited planet 20 years prior. After contacting the only survivor of the crash, the captain is faced with a deeper mystery: why does that survivor refuse to leave the planet he crashed on, and why does he so badly want the captain to leave immediately?

The Creation of the Humanoids (1962). An obscure little gem about the consequences of artificial intelligence and robotics. It's performed more like a stage play than a movie. Don't expect action scenes, do expect lots of dialog.

2001: A Space Odyssey (1968). Legendarily slow-paced but a true classic. Parodied and referenced in pop culture to the point where it might not surprise you at all. But if you haven't seen it in full from start to finish it's well worth doing so. A minor plot point is that the USSR and USA are allies, peacefully collaborating on space exploration. The only Soviets in the movie are a group of radio astronomers that one of the American main characters amiably chats with on a civilian space station that's jointly operated by the USA and USSR. Not explicitly leftist but definitely not reactionary. Oh, and if you partake of cannabis from time to time, this is definitely a movie to watch after partaking. With a top-up during the intermission.

Beyond the Black Rainbow (2010). Set in the 1980s, a girl whose name can be shortened to "El" is being held captive in a psychic research facility. She's being confined there by a creepy father-figure who has dark designs on her, and also on her telepathic and telekinetic abilities. (And if that sounds suspiciously familiar, just compare production dates.) Slow-paced, it's an interesting blend of old-style and new-style filmmaking techniques.