I haven't seen
Leviathan either but was deeply impacted by
Vozvrashcheniye. It's one of my all time favorites, was lucky enough to see it in the cinema and it blew me away.
Speaking of soviet era masterpieces, one that may not be very widely known (I think) and that also blew me away:
https://en.wikipedia.org/wiki/The_Ascent_(1977_film)