Best sci-fi is from the 1940s and early 1950s. Everything after that is just action thrillers in space...
I strongly disagree.
So many science fiction stories, from the 70s (Logans Run), to the 80s (Blade Runner) to the 90s (The Matrix), had deeper commentary on social issues. In fact very little of what was regarded as the best Star Trek movie, IV-The Voyage Home took place in space, and it was loaded with lessons.
I think 1977's Star Wars was the turning point. Prior to, sci-fi morphed from incredible movies like War of the Worlds to bland, cerebral fare like Solyent Green. After Star Wars, sci fi grew in both technical prowess and story line.