I have a few favorites, actually.
Star Trek II: The Wrath of Khan is on the list, as is The Black Hole, 2010, the original Alien and Terminator movies, and of course the Star Wars original trilogy.
Sci-Fi Movies Thread
a month later
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- Alien Quadrilogy (with Sigourney). Prometeus and Covenant were quite a disappointment with all that overly complicated genetic manipulation/biological warfare/philosophical nonsense.
- Terminator 1 & 2. The other movies are just waste of time and money.
- Fifth Element. Really cheerful for a Sci-Fi genre, flying cars, polluted city and all those cliches turned to 11.
- Total Recall. This movie never gets old with cheesy 80's style.
- Predator. Do I need to explain?
- They Live. Some nice points about corporate world with lot's of funny stuff going on.
- Running Man. Arnold in spandex going on rampage in a futuristic killing game show.
- Blade Runner. Set design that started cyberpunk in movies and games.
- Avatar. Most amazing CGI and 3D I've ever seen that looks better than anything from 2009 to this day.
- Thing. Horryfying to this day with some great arctic atmosphere.
- I Come in Peace/Dark Angel. 80's style that never gets old.
...and then everything turned into boring CGI nonsense and proper sci-fi was replaced with janky, repetitive super hero craze. 
MattX Alien Quadrilogy (with Sigourney). Prometeus and Covenant were quite a disappointment with all that overly complicated genetic manipulation/biological warfare/philosophical nonsense.
I guess you are talking about the Alien Duology. Alien and Aliens are indeed two excellent movies. Just like Terminator, there are only two of them. 
MattX Intriguing list. The Thing is still a great great movie that no one ever mentions. Even Escape from new York is still watchable. For a Bruce Willis sci-fi movie I think 12 Monkeys far superior over 5th Element.
Personally, for me, likely in the tiny minority, I found Avatar to be a horrible watch, painful, so bad I just didn't care especially the acting and feel-good plotline. Simply an awful experience that I'll never do again. I'll tell you how I really feel about it another time.
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kyrios I liked Alien 3 and Resurrection. They were far more entertaining than most of crap of today. I got all (5 + Prometheus) Aliens movies on DVD except Alien vs Predator ones, because I find them kind of ridiculous.
brent I can agree that Avatar wasn't that good on the plot side, but visually it was mindblowing. Now we are used to bombastic CGI, but back in 2009 in the cinema it blown my mind how beautiful the created world was and I've seen it in 3D then. Later when I watched a 3D movie I wasn't impressed. The colors, the depth were mindblowing. Even now on a huge 4K TV it looks great. And if you have oportunity to watch it on a UHD + 3D capable gear it's still impressive. And the plot... It's just Pocahontas in space. Nothing super-inteligent, but if you know other flicks by Cameron you know that his movies are more into visual experience than anything else.
I just noticed this article this morning, and thought perhaps there are more Dune fans here. It's apparently a new take on an oldie but a goodie.
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I usually like David Lynch's movies, so I did enjoy Dune although I saw it a long time ago and I would certainly be disappointed if I had to watch it again.
I also enjoyed the video games Dune and Dune 2.
I can't wait seeing the new Dune. No doubt it will be a great movie, the trailers are promising. Denis Villeneuve didn't disappointed me with Blade Runner 2049 and Hans Zimmer soundtracks are always great.
Perhaps one day I'll read the books...
kyrios Perhaps one day I'll read the books.
You won't be disappointed. When it comes to SciFi, Herbert was a god!
The figure that's usually quoted for a movie based on a book, is that it requires one minute of motion picture to express one page in the book. I'm not sure if I believe that figure, because in my experience it seems more like 2 or 3 pages per minute. Obviously, it depends on the movie.
And just as obviously, a huge epic can't be done with a single movie, without leaving something out. Sometimes a lot needs to be left out. The moral of this story is, "If you enjoyed the movie, you'll probably love the book(s)."
In my own experience, I prefer the book(s) for content, and the movies for presentation values.
I read the book quite some time after I'd already seen the Lynch adaption, and really, really enjoyed it. Funnily enough I didn't have (m)any images from the movie in my head while reading it, but conjured up a completely different world (within the framework of course). Maybe because the movie wasn't fresh on my mind anymore.
I will definitely go see the new movie too, even though something feels off to me looking at the trailers. Can't put my finger on it though.
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WetGeek The moral of this story is, "If you enjoyed the movie, you'll probably love the book(s)."
S. King is the exception for me but I otherwise agree with you on that point. I find movie adaptations of his books much better than the hard (for me) experience of reading them. (IMHO of course). edit: clarity
Movies based on S. King's books are mostly not very faithful to the source material. It's because they would be just too weird or too long. I watched 'Thinner' some time ago and this movie was bizarre. Some scenes looked kind of retarded. 
WetGeek I prefer the book(s) for content, and the movies for presentation values.
What I didn't say in that post, and came back too late to edit it, was that I'm able to enjoy one without distracting anything from the other. I approach both movies and the books they're based on with a different set of expectations, and thus I'm rarely, if ever, disappointed.