Definitely a classic is “Solaris” (1972 film) by Andrei Tarkovsky. It has a mix of sci-fi and drama, but I can’t tell more without spoiling it. It definitely makes you think … Also other Tarkovsky films have been praised, like “Stalker” that you mentioned. So maybe check out his filmography?

Edit: Something completely different is “The Mystery of the Third Planet” (1981) by Roman Kachanov. It is an animated sci-fi film for children, but also fun to watch as an adult because of the absurd storyline and humour.

Other films I have in mind, too, but don’t remember their exact titles now. Will edit this post, if I recall … Edit: You might want to have a look at the Wikipedia list differentiating by age and/or genre: https://en.wikipedia.org/wiki/Cinema_of_the_Soviet_Union