Since the blockbusters are already well-known, here are some lesser-known titles.
They Live - I saw it on VHS in the 80s; it's very different watching it now.
Dark Star - Another Carpenter movie, with Dan O'Bannon, who was working on his own project at the time, "Star Beast". That was a lame title, so it was shortened to "Alien". Interesting to see the similarities between Dark Star and Alien.
The Thing is better every time I watch it.
Scanners is sort of SF, sort of horror, but pretty good.
Death Race 2000 for some interesting 1970s commentary on media-run society
The Arrival came out around the same time as Independence Day, different take on alien invasion
The Andromeda Strain (1971) was a great adaptation of the Crichton book.
(TV) I've always been partial to Gerry Anderson's UFO series. Greatest opening theme in TV history, and an interesting view of the year 1985. Space 1999 was the one I watched every week in the 70s, but UFO was more fun to watch.