I have had conversations with three people recently (one being my own farther!) who have said they don’t like Sci-Fi. When asked why they all came up with the same vague answers about it not being real, or realistic and they can’t suspend disbelief if the concept is too far from reality. But none of them had a problem with unrealistic plots in other films if the film was gunny or exciting. When challenged they all came up with a Sci-Fi film they actually liked, but hid behind things like, “but its funny” or they like the star. I didn’t intend to turn into a ardent defender of Sci-Fi or any other genre, but firmly believe there are two types of movie; good and bad, this is true regardless of genre.
With this in mind I have come up with my ultimate list of ten(ish) Sci-Fi films everyone should watch. I have taken out some of the more challenging movies: 2001: A Space Odyssey (1968), A Clockwork Orange (1971), Solaris (1972), Twelve Monkeys (1995), Eternal Sunshine of the Spotless Mind (2004); anything animated that they may try and dismiss as kids films: Akira (1988), WALL·E (2008); and anything silent: Metropolis (1927). I have also taken out anything with serious baggage like Star Wars and Star Trek. We are left with my Must See Sci-Fi list:
Alien (1979) & Aliens (1986)
Blade Runner (1982) 
The Thing (1982)
The Terminator (1984) & T2 (1992)
Back to the Future (1985)
The Matrix (1999)
Donnie Darko (2001)
Serenity (2005)
Children of Men (2006)
Inception (2010)
Fandango Groovers Movie Blog 
Not a huge fan of The Thing, and haven’t seen Serenity (or Firefly) yet, but I love everything else here. Great list!
Serenity is brilliant, check it out.
Awesome list, love BladeRunner & Alien, also I rate Gravity, Universal Soldier & M.I.B
Gravity very nearly made the list.
Good list of essentials. Of course The Thing may be a tough one for ones sensitive to what at the time was considered excessive gore. My brother and I rented that as our first movie when my dad first purchased a VCR. He almost returned it after watching part of the movie with us.
Not sure it deserved its X (18 on video) rating it got here in the UK but it certainly has its gory moments.
It got pulled from the local theater in Corte Madera, California after three days because some parents complained though having not even seen the film themselves . The couple of kids that managed to see it became celebrities and were pestered by their peers for all the best bits.
You need some older films in that list, its ignoring entire decades of definitive sci fi films, films like Forbidden Planet and Soylent Green and so many others.