Scifi movies are fun:
I have probably seen most of the good films in this genre and many of the bad as well. My favorites that come to mind are:

1. War of the Worlds (1953)
2. Alien
3. Aliens
4. The Thing (195-)
5. Invasion of the Body Snatchers (195-)
6. Jurassic Park
7. The Day the Earth Stood Still
8. The Terminator
9. Blade Runner
10. Independence Day