Have you seen Jodorowsky's Dune? It's a very cool recent documentary about a failed (but possibly epic) adaptation of the Frank Herbert novel. You can pick up a few nice recommendations from it too. Alien is pretty obvious but Dark Star is often overlooked.
A great widescreen Technicolor sci-fi with pioneering electronic sound design is Forbidden Planet. You might as well watch the French animated classic Fantastic Planet too.
Silent Running is a hippie space movie directed by acclaimed SFX artist Douglas Trumbull. It's a little dated but still worth a viewing.
Maybe you already know about these, but they're a few basics that came to mind. I'll try to add some less obvious names later.
