George Lucas' Star Wars changed the entire movie industry. It almost single-handedly (along with Jaws) created the "Blockbuster Age", which Hollywood is still sitting in. It also revolutionized the genre of science fiction; what was often thought of as either silly Saturday morning fluff, or a great way to make an allegorical film with a message, was now The Big Way To Make Money.
Sci-fi has continued to evolve, or devolve, ever since (Lucas himself couldn't make lightning strike often, with his horrible Star Wars prequels tarnishing the great original sequels). But I like to watch how sci-fi filmmakers worked their magic before Lucas changed everything. Often they were more innocent and cute — other times, they were far more dark and violent. The special effects were rarely as good as what Lucas achieved, but sometimes the brains behind the brawn were far greater.
This is a very not-comprehensive list of my favourite pre-Star Wars (so before 1977) science fiction films. There are literally hundreds more out there, and some very famous ones I haven't yet seen (like Forbidden Planet, Soylent Green, Logan's Run, etc). But from what I've seen, these are some damn good ones, and can stimulate the imagination as much as impress with their visuals and creativity.