I’d have Gattaca in the top 5. I’ve not seen Children of Men, I really have to.
I’m excluding Science Fiction movies that are set with futuristic elements, but are at their heart action movies. So Alien, Aliens, Predator, Terminator II, Empire Strikes Back, Wrath of Khan, etc. A few action movies, such as Edge of Tomorrow, Robocop, Total Recall, The Matrix would still qualify as having serious science fiction concepts as a core element, but sit outside the top 5 for mine.
Watchman had good elements, but the directing and editing was awful. Kept too true to the graphic novel, stuff stayed in that should have been cut.