Not necessarily what I think is the best as that list fluctuates a lot, but if someone asks what I would recommend, this would probably my shortlist:
Children of Men - Probably one of the most important movie of the genre
Blade Runner - Saw the film quite young and back then my whole definition of sci-fi are people on spaceships shooting lasers. Didn't like it. Took me a few years and re-watch to understand the movie (and appreciate the soundtrack).
Attack the Block - Because its fun. For me it has the same energy as Goonies and Shaun of the Dead. Totally re-watchable.
Annihilation - I would admit this is probably due to recency bias but there's not a lot of films in the New Weird genre and part of me is just glad this movie was made.
Minority Report - It's a blockbuster sci-fi action film first and foremost but it does posit some ethical and moral quandary that would make you think if you want to dig deeper. Still a solid popcorn flick if you don't. Most people remember this film because of the gesture tracking tech but damn, they predicted pesky targeted ads.
Eternal Sunshine of the Spotless Mind - Because its poignant. It's a sci-fi that is not about the science but about people and relationship. Most good sci-fi aren't about the sci-fi anyway, it's about the characters.
Alien and The Thing - Horror and sci-fi mix together well. Unpopular opinion: both films kinda feel dated but it's not their fault but because a lot of horror films through the years have done the same jump scares ad nauseam although to be fair some have innovated and subverted it. As pioneers though, barely any movie can touch these two.
Back to the Future - Same as Attack the Block, it's fun, it's entertaining. The characters are likeable. The movie knows what it is and movies doesn't need to always have some deep introspective philosophical message for it to be good.
Probably a few more that I'm too lazy to write down at the moment.