I'll try for a bit of a more mixed selection for you from a wider range over time.
Cloverfield
Quite a strange film in a Blairwitch style POV handcam style, but also quite emmersive. It's one of those films that you watch wondering if you are enjoying it, but continue as it is quite fascinating. But I don't think you would watch it twice so hire or DL it.
IronMan - great film. Although lots of missed opportunities with the story.
Fifth Element - still one of my favourite Sci-Fi films.
Star Trek - The new Star Trek film was moans about by many die hard Star Trek fans, but for entertainment, big screen action, and some great characterisations it is brilliant.
Screamers - Great Sci-Fi Horror. Takes the Terminator idea further.
Dark Star - a Cult Classic Sci-Fi film that is a must see. The scene where one of the crew is arguing the existence of god with a bomb is worth it alone.
The Thirteenth Floor - A thought provoking take on virtual reality.
Sunshine - Great cinematography that really needs a big screen and HD.
Event Horizon - I know it's been mentioned, but it is good Sci-Fi Survival Horror.
Evolution - Great tongue in cheek Sci-Fi alien insects invade earth film. If you liked Starship Troopers you will enjoy it.
Robocop - The original is still the best.
And finally, the original Star Wars trilogy and all Star Trek films. Must have for any Sci-Fi session.