For starters, the Quatermass series from Britain. They appeared as serials on TV, and were re-made as movies:
The Quatermass Xperiment (sic), realeased in the US as The Creeping Unknown
Quatermass 2, released in the US as Enemy from Space. Noth good films about offbeat alien invasions. The only problem, is that film film versions starred American Brian Donleavy as Quatermass, who looks and sounds more like an American gangster than a British professor.
Quatermass and the Pitt AKA Five Million Years to Earth – really good SF piece about finding a long-buried Martian spaceship while fixing up a London Underground station. Like 2001, released the same year, it involves an alien aretifact which was involved in human evolution, and which is still working. And this time the very British Andrew Keir plays Quatermass. Highly recommended.
I don’t recommend later entries in the Quatermass series, though.
Then there are Jerome Bixby’s films of the 1950s:
It! The Terror from Beyond Space – really good film about a Monster Loose on a Space Ship. The first film using this trope, which I’;m convinced Alien largely ripped off from this film. It had been done in fiction by A E Van Vogt earlier, but Bixby’s film isn’t really a rip-off of Van Vogt. Again, highly recommended.
The Lost Missile – really rare. A missile from some undisclosed country (pretty obviously Russia) is in low earth orbit and causing havovc by somehow creating great heat and devastation. It has to be destroyed before it destroys the world.
But ignore Bixby’s other 1950s film, The Curse of the Faceless Man, which is about a Pompeiian mummy come back to life. Bixby, by the way, was responsible for the story used in that Twlight Zone episode "It’s a Good Life, where Billy Mumy can do anything with his mind, and sends people he doesn’t like Out To The Cornfield. He also rewrote the screenplay for Fantastic Voyage. A film version of his **The Man from Earth[/B[ came out about 15 years ago, and is worth seeing.
The Last Mimzy – too upbeat, and not entirely faithful, but we need more Henry Kuttner/Catherine L. Moore-based movies. This one is based on their “Mimsy were the Borogoves”
Timescape – another film based on a Kuttner/Moore story, this time “Vintage Season”. Once again, the movie’s ending is more upbeat than the story’s. But worth watching.
Panic in Year Zero – Really low budget film about father Ray Milland getting his family to safety after the Russkies (unnamed, again) nuke LA. Clever low-budget film making.
Robinson Crusoe on Mars – so good that it’s available from Criterion Video. Despite some of its inanities (who the hell brings a gun to Mars? Well, except those guys in It! The Terror from Beyond Space), this is surprisingly good. It’s also (I think) pretty well known. Or it should be.
Creator – A movie about cloning that doesn’t involve Awful Stuff happening, or rapidly-maturing clones, and with genuine wit. Peter O’Toole stars as a Nobel Lsaureate trying to clone his long-dead wife. Accused of being too soap-opera-ish, but I love it. It really does feel like life in acadmia.
Android – very weird 1982 flick about Klaus Kinski as a scientist making illegal androids in his spaceship lab. Not great, but better than you’d expect.
Creation of the Humanoids – the fuilm quality is rotten and the acting incredibly wooden, but this film has something to it. It’s far rem,oved from most cliche SF films of the time.
The Hidden – not sure if this is really not well known, but it certainly isn’t as well-know as it deserves. 1982 film starring Kyle McLaughlin playing a detective very much like the one he played in Twin Peaks, chasing down an alien, and this fiolm feels as if it explains how he got that way. It’s as if they filomed Hal Clement’s Needle. Very highly recommended.
The Thirteenth Floor – when The Matrix came out, it eclipsed the two other Virtual Reality movies of that year – David Cronenberg’s XistenZ and this one. Both are worth watching, but I prefer this one, based on Daniel Galouye’s 1964 SF novel Simulacron-3, which essentially invented the VR novel. It was filmed once before as a TV film in Europe. Very higghly recommended.