I feel that Event Horizon is the most effective science fantasy movie produced. I've seen it once, which was enough to leave an indelible mark on me.

Event Horizon combines the supernatural with some elements of vaguely plausible science fiction. The movie comes together in a way that traces back to the present-day with a pretty bright line, especially in prop and spaceship design. It keeps you connected and then dumps the fantasy element on you hard. Disturbing but well done stuff. Deeper nerd-cut, it's a 40K prequel, if only unofficially.