John Scalzi has rattled our cage, so here we go.
The one movie that impressed and affected us the most, the one that we catch each time it comes on, has to be Close Encounters of the Third Kind. Steven Spielberg showed us a marvelous portrait of a man who is touched by the inexplicable and whose whole life gradually falls apart as he tries to fathom what has happened to him. We witness his wonder and confusion as he is compelled to recreate his experience, and the effect on his family and neighbors. The end of the film rewards him, and those who were alike affected, for reliance and trust in his feelings and intuitions.
Our favorite runner-up is The Day the Earth Stood Still, one of the first of the early atomic-age efforts to seek to point out the unintentional folly of humankind.
Our three favorite cycles would have to be the Alien series, the Terminator series, and the Star Wars films.
As far as the fantasy genre goes, the Lord of the Rings trilogy wins hands down. The Harry Potter series comes in a close second.
Our favorite worst genre movie of all time is Manos: Hands of Fate, a truly awful effort by a man named Hal Warren who wanted to make a standout on a shoestring and succeeded admirably, if not in quite the way he wished. If you can catch the MST3K show on which it was featured, pop some corn and get ready for a rousing good time.
I have to agree with John's estimation of Ripley as the most successful, well-developed character in science fiction. Throughout the series of films, and especially in Aliens, the sheer guts of this woman and the fearlessness against exceedingly long odds that she displays are inspiring.