This is in response to an Entertainment Weekly article which identified the top 25 sci-fi TV and movie phenomenoms over the last 25 years.
I have a very detailed version of this I intend to expand upon as I improve on this, but here's what I would say are the ten most definitive sci-fi films of all time and when I get the time, I will expand upon this more. So consider this incomplete without explanations:
1. Star Wars, George Lucas, 1977
2. Metropolis, Fritz Lang, 1927
3. 2001: A Space Odyssey, Stanley Kubrick, 1968
4. A Clockwork Orange, Stanley Kubrick, 1971
5. Close Encounters of a Third Kind, Stephen Spielberg, 1977
6. Blade Runner, Ridley Scott, 1982
7. Brazil, Terry Gilliam, 1986
8. Matrix, Wolchowski Bros., 1999
9. Alien, Ridley Scott, 1979
10. Back to the Future, Rob Zemeckis, 1985
10 other films of note: The Day the Earth Stood Still, Invasion of the Body Snatchers, War of the Worlds (original), 20,000 Leagues Under the Sea, The Day After Tomorrow, Independence Day, Aliens, Minority Report, X-Men, E.T.
10 great Sci-Fi TV Shows I recommend: Lost in Space, Star Trek, Star Trek: Deep Space Nine, Seaquest DSV, Futurama, Sliders, Quantum Leap, Firefly, Andromeda, Heroes
10 sci-fi books: Dune; I, Robot, Hitchhiker's Guide to the Galaxy, Ender's Game, The Time Machine, Journey to the Center of the Earth, 20000 Leagues Under the Sea, Farenheight 411, and hmmm, can't think of any other ones.