Now this one is fun. Once again I'm excluding Doctor Who because that has already had its own missing show category. When I first thought about this question I assumed I would be picking Nigel Kneale's The Road which was a ghost story from 1963 with an ingenious twist in the tale of an 18th century village haunted by strange sights and sounds. You can find the script for it on the DVD of The Stone Tape, that's if you can still find a copy of The Stone Tape.
However another idea occurred to me. When I was young I was a huge fan of Isaac Asimov, and of his robot stories in particular. I am pretty sure that The Caves of Steel was my favourite novel as a teenager. I loved the odd couple pairing of a human detective with his robot partner as they try to catch a murderer in an over-crowded city of the future.
While researching shows for us to cover on British Invaders I discovered that there was a BBC adaptation of the novel in 1964 which starred Peter Cushing as the detective Elijah Bailey and John Carson as R. Daneel Olivaw, and it was adapted for TV by none other than Terry Nation. Sadly only one or two clips of the production have survived but wouldn't it be great to be able to watch that show?