"Sci-fi" is such a huge field

. Just to decide what sci-fi actually
is would take its own thread: hard sci-fi vs soft sci-fi, plot sci-fi vs aesthetics sci-fi, technological sci-fi vs philosophical sci-fi... etc. Given that the genre has been going strong since the 50s and the known "light entertainment" genre literally grew out of it, I'd say it'd be at least ambitious to name 10 for all times.
I'll offer the list of those titles that have shaped my personal taste over the years. I know there are more refined / higher grossing / more acclaimed ones out there. These somehow got stuck in my mind and I find myself re-watching them from time to time.
Star Wars - The Empire Strikes Back
In school, we all agreed who could be Luke or Han, and since we did not ever let girls get close to our territory nobody was Leia. When the 2nd movie came to town, my dad reluctantly agreed to take me to the cinema. I sat the whole movie through without swallowing or blinking once and was coughing and tearing for an hour afterwards. I've never wanted a hoverboard. I am a lightsaber fan. For me canon SW has 4 movies only.
Tron
My first pocket money movie ticket and my first movie experience alone. I was so blown away that I had to spend the rest of my wealth to bring all my beloved ones to the subsequent screenings. Even today I enjoy re-watching it form time to time. While the later ones in the series are more polished visually they simply can't even compare to the depth of the original one's vision.
Terminator
Saw it on a pirated VHS tape that my dad rented on the black market for me. This movie arose a deep primal fear in me. "It can't be bargained with. It can't be reasoned with. It doesn't feel pity, or remorse, or fear. And it absolutely will not stop... ever, until you are dead!" - nothing ever came close to this threat. Not even the xenomorph. When the T-800 performed surgery on itself my dad swore never to let me pick a movie ever again. If you like this, I also recommend Hardware.
THX-1138
I had the pleasure to watch this when I was compiling my collection of debut movies from well known and later successful directors. THX-1138, to me, is the perfect example of unpolished potential. It has almost everything sci-fi as a genre offers: technological futurism, social dystopia, introspective awakening, quest for freedom, post-modernism in buckets and perfectly experimental music. Every single scene is oozing sci-fi aesthetics. The only issue with this movie is that it feels like drinking condensed milk. It is just
too rich.
Akira
In the 80s I saw a snippet of this on TV and my jaw dropped. For a very long time I had zero idea what this amazing animation could be until decades later - already in the Internet era - I came across it in the cinema. My first anime, and it immediately closed a very important loop for me. As sci-fi it's excellent. While I'd love to nominate LOTGH instead for its absolutely unparralleled grandeur and its unalloyed romanticism it isn't exactly a movie with its 100+ episodes.
Moon
Another one from my directorial debut collection. I went to the cinema completely unassuming and was more than pleasantly surprised. While the movie does build out of already well known tropes, somehow it delivers a completely fresh and unexpectedly smooth mix. This is the best kind of "more than the sum of its parts" scenario.
TENET
After Inception I already kept my eyes peeled for whatever Nolan directed. I heard it was intense, but nothing prepared me for the sensory assault. It felt like a too demanding fitness workout - but for my brain. I watched it repeatedly just to make sure every single scene indeed clicked the way it at first seemed to click. It prompted discussions and re-watching sessions over and over again. I do love Interstellar, but this one had a greater impact on me.
The Matrix
How to say "comicbook culture is back" louder than this? If there ever was a movie that could slip into the realm of global conspiracy theories this was the one. Hell, even I am convinced we live in it. Fusing nearly everything I love in light entertainment and plating it in a perfectly balanced package with uforgettable charisma, music, visuals... and there is Kianu. I simply refuse to believe the Watchowskis are unable pull another one off.
Escape from New York
Could have been Running Man, or The Thing, but somehow Escape from New York was the one that musically resonated with me so much that even decades later I enjoy listening to the many forms of Synthwave. Almost all Carpenter creations are epic, somehow this one was the title that me and my friends loved the most. If you feel particularly brave, try Carpenter's Dark Star!
Blade Runner
If I had to pick my favourite sci-fi movie, this would be it. All-in-all, Blade Runner is the one that comes closest to 100% for me. The cinematography, the world building, the narrative, the dialogue, the atmosphere, the props and locations, everything clicks with me. I find the scenes hauntingly beautiful. I think the casting is perfect. I am grateful I got to see Sean Young, Harrison Ford and Joe Turkel in the same room. I remember it by heart. I have every official and fan edit version of this movie and of the soundtrack.
I've seen things... and Blade Runner stays one of my most cherished movie experiences.