Yes ! Aliens had 9 plans to take over the earth. This is 9 more than BP had for it's DWH disaster. The aliens only error was to choose the plan to raise an army of dead humans; clearly, us live ones can make things bad on our own !
My favorite Silent Running. just because that would be me.
Every single Roddenberry series.
Every single Star Wars (I still try to levitate a rock now and then, some day........
The Lathe of Heaven (PBS version)
Reading everyone's list I find it interesting that nearly everyone has a "list", but not a specific favorite. Maybe that is particular to engineers, who knows, but for us, you can either make the list or not, but we don't have a defacto all-time favorite. Same goes for me, there are aspects of several sci-fi movies that "make the list", but I can't bring myself to pick 1 favorite and rule out all of the other great movies.
If only because it seems to be the very first movie to star an extremely young Leonard Nimoy, I vote for "Zombies of the Stratosphere," an otherwise dreadful movie serial with a plot that was so convoluted that it deserves the MST3K treatment. For good sci-fi, look to Blade Runner, which was based on a Phillip K. Dick novel titled "Do Androids Dream of Electric Sheep?" The cinematography and set design on this picture (starring Hrrison Ford, Rutger Hauer, Sean Young, Edward James Olmos, Darryl Hannah)is really amazing. See it on the biggest screen you can.