The classic BBC anthology series. Only a few still exist, but when they first aired I watched them all. Best TVSF I've ever seen, because they treated the literary sources with the same fidelity and presumption of serious intent given to any classic adaptation. I've no doubt they'd appear creaky and flawed if I saw them now... but in their time and their context, they were damn-near perfect.
Of course, 'still exists' is a pretty flexible term, as evidenced by the truncated state of Little Black Bag, the Kornbluth story of which only a fragment survives.
Look at a 60s Twilight Zone now and it probably looks even better than it did back then, given that it's on stable and detailed 35mm film, probably remastered from an uncirculated print, and that both telecine scanning and TV sets have improved so much.
The BBC shot on tape, which it tended to wipe and re-use after a couple of showings. 16mm telerecordings were photographed off a screen for export to markets undiscriminating enough to accept them as broadcast quality, and these are what tend to survive.
It's a bit like burning the Book of Kells and keeping a photocopy. Where original tape does survive, it serves as a cruel reminder of what's been lost... Nigel Kneale's The Stone Tape now looks far better on DVD than it ever did on our 70s TVs. One falls into the trap of thinking that because old British TV looks so bad, that's what we must have put up with back then.
The BBC now has a restoration unit, economically justified by the revenue potential of old Doctor Who material and staffed by people who you imagine would probably do the same work for nothing if you took their funding away. One of the tools they've developed is Vidfire, a means of taking the old telerecordings and interpolating false frames to restore some of the 'video look' of the original material. They can tweak the contrast, clean up frames, digitally remove hairs, stabilise the jitter of shrunken film, and are generally elevating technical turd-polishing to the status of an art.