Interesting sci-fi movies BY TOPIC?

My favorite, ML-based movie recommendation website is (temporarily?) down.

The main reason I like SF (movies) is they explore intriguing ideas. HN has had a number of threads about favorite SF movies, but they tend to repeat the same movies. And some topics are very repetitive, for instance it's hard to find a time-travel movie which does not show the grandfather paradox.

In this post, we can take advantage of threaded nature of HN and create a list of most interesting SF movies by topic. -----------

Rules:

1. Top-level comments are for topics ONLY. I suggest not to upvote any of them, my intent is not karma fishing.

2. Only mention individual movies in comments of 2nd level and later. (index 1 when starting from index 0)

3. For each movie, try to write a single sentence describing why this movie shows an interesting angle.

EDIT: go ahead and create a category if a movie focuses on a different theme!