• The Adventures of Buckaroo Banzai Across the 8th Dimension! -- A fun movie, to be sure, but ultimately disappointing, and I'm not sure where its significance lies.
  
• Akira
  
• Alien -- Really a horror film, but very good.
  
• Aliens -- My favorite of the Alien series.
  
• Alphaville
  
• Back to the Future -- I'd dispute the inclusion of this piece of schlock.
  
• Blade Runner -- One of the best, ever. Many layers here.
  
• Brazil -- Was the director's cut ever released?
  
• Bride of Frankenstein -- But not Frankenstein?
  
• Brother From Another Planet -- I've tried to like it, without success.
  
• A Clockwork Orange -- A difficult film to watch, but very good.
  
• Close Encounters of the Third Kind -- What's to say?
  
• Contact
  
• The Damned
  
• Destination Moon
  
• The Day The Earth Stood Still -- A classic, one of the best.
  
• Delicatessen
  
• Escape From New York -- Huh? Scalzi is joking here?
  
• ET: The Extraterrestrial -- Very entertaining.
  
• Flash Gordon: Space Soldiers (serial) -- Update: In researching the release year for this, I realized it was one of the Flash Gordon serials I had seen as a kid.
  
• The Fly (1985 version)
  
• Forbidden Planet -- Almost, but not quite, sunk by its Freudian b.s.
  
• Ghost in the Shell -- I'm totally unfamiliar with this 1995 anime film, also called Kôkaku kidôtai
  
• Gojira/Godzilla
  
• The Incredibles -- You've got to be joking! One of the most significant science fiction films of all time!?! I have no doubt it's probably a very entertaining film, but, come on.
  
• Invasion of the Body Snatchers (1956 version) -- Another classic.
  
• Jurassic Park -- Never got around to seeing it, and I don't particular feel like I'm missing anything.
  
• Mad Max 2/The Road Warrior -- I've seen large portions of it, and I'm certainly aware of its pervasive influence in terms of style.
  
• The Matrix
  
• Metropolis -- Worth watching again.
  
• On the Beach -- I have a copy of this, but haven't watched it yet.
  
• Planet of the Apes (1968 version) -- I've studiously avoided seeing this, or any of the others in the series.
  
• Robocop - (snicker)
  
• Sleeper - Funny, but I would dispute its inclusion here.
  
• Solaris (1972 version) -- Have it, haven't seen it.
  
• Star Trek II: The Wrath of Khan -- All the Star Trek films have been disappointing to one extent or another. Probably the most entertaining of them has been Star Trek IV: The Voyage Home (the one about the whales), but Wrath of Khan was surely the right one to include on this list.
  
• Star Wars Episode IV: A New Hope
  
• Star Wars Episode V: The Empire Strikes Back -- These two are clearly the best of the 6 Star Wars films.
  
• The Stepford Wives -- Which version is Scalzi referring to, the 1975 original or the 2004 remake?
  
• Superman -- Eh.
  
• Terminator 2: Judgement Day -- Another I've seen large parts of, but was never interested in seeing the whole thing.
  
• The Thing From Another World
  
• Things to Come -- Disappointing. Influential, but not very watchable.
  
• Tron -- I've seen a bt of it.
  
• 12 Monkeys
  
• 28 Days Later -- Also waiting for me to watch it.
  
• 20,000 Leagues Under the Sea -- I've seen some of it.
  
• 2001: A Space Odyssey -- Hugely influential, at least until Star Wars came around. There's really nothing like it.
  
• La Voyage Dans la Lune -- Update: I belatedly realized that this was the 1902 Georges Melies film, which I've seen
  
• War of the Worlds (1953 version)

Films that probably should have been included:

• Altered States (1980) -- An "inner space" movie.


• Dark Star (1983) -- Very influential, if not nearly as funny as its supposed to be (it might help to watch it stoned).


• Fahrenheit 451 (1967) -- A flawed film, but very stylish and strangely affecting.


• The Man Who Fell to Earth (1976) -- Mysterious and fairly non-linear.


• THX 1138 (1971) -- George Lucas's dystopia is quite good looking, and not bad to watch.

Note that "most significant" does not mean "best" or "most popular" or even "most influential." Some of the films may be all three of these, but not all of them are -- indeed, some films in The Canon aren't objectively very good, weren't blockbusters and may not have influenced other filmmakers to any significant degree. Be that as it may, I think they matter -- in one way or another, they are uniquely representative of some aspect of the science fiction film experience.

Suitably vague, allowing him quite a bit of leeway to include personal favorites and exclude those he disliked. Without a more rigorous definition, it's difficult to argue with his choices.

Data: Here's Scalzi's list of films with their US date of release:

1902 Voyage Dans la Lune, La
1927 Metropolis
1935 Bride of Frankenstein
1936 Flash Gordon: Space Soldiers (serial)
1936 Things to Come
1950 Destination Moon
1951 Day The Earth Stood Still, The
1951 Thing From Another World, The
1953 War of the Worlds
1954 Gojira/Godzilla
1954 20,000 Leagues Under the Sea
1956 Forbidden Planet
1956 Invasion of the Body Snatchers
1959 On the Beach
1962 Damned, The
1965 Alphaville
1968 Planet of the Apes
1968 2001: A Space Odyssey
1971 Clockwork Orange, A
1972 Solaris
1973 Sleeper
1975 Stepford Wives, The
1977 Close Encounters of the Third Kind
1977 Star Wars Episode IV: A New Hope
1978 Superman
1979 Alien
1980 Star Wars Episode V: The Empire Strikes Back
1981 Escape From New York
1981 Mad Max 2/The Road Warrior
1982 Blade Runner
1982 ET: The Extraterrestrial
1982 Star Trek II: The Wrath of Khan
1982 Tron
1984 Adventures of Buckaroo Banzai Across the 8th Dimension, The
1984 Brother From Another Planet
1985 Back to the Future
1985 Brazil
1985 Fly, The
1986 Aliens
1987 RoboCop
1988 Akira
1991 Delicatessen
1991 Terminator 2: Judgement Day
1993 Jurassic Park
1995 Ghost in the Shell
1995 Twelve Monkeys
1997 Contact
1999 Matrix, The
2002 28 Days Later
2004 Incredibles, The