¿La mejor película de ciencia-ficción que hayas visto?

Tough one! Due to its significance and good balance between script, visual effects and cultural phenomenon that it sparked, I would go for "The Matrix (1999)". Since there are several topics in science-fiction, I would like to remark the most important films in each one.

virtual worlds: "Inception (2010)"
space opera: "Star wars (1977)"
time travel: "Twelve monkeys (1995)"
aliens on Earth: "District 9 (2009)"
artificial intelligence: "Blade Runner (1982)"
and so on...

Ask me anything. I deal better with "why" and "how" than with "what".