What's the greatest science fiction action movie of all time?

Tons of science fiction movies feature crazy stunts, fight scenes, chases, and explosions. And plenty of action movies have some science-fictional ideas buried in their plot twists. But what's the absolute greatest science fiction (or fantasy) action movie ever made?

Please include a photo, movie poster or clip of your chosen movie. Plus the movie's title, and maybe a bit about why it's so frakkin great.

Top image: Terminator 2: Judgment Day.