Answer: Dr. Who

As far as sci-fi series go, Dr. Who is a veritable Methuselah. The show, focused on The Doctor (a humanoid alien with extraordinary powers) and his intergalactic adventures, first aired in 1963 in Britain. It went on to run for 26 seasons, ending in 1989. In 1996, there was a TV film, and then, in 2005, the show was resurrected. The show is, as of this writing, still being broadcast and has aired over the course of original and the current ongoing runs, a staggering 840+ episodes.

If you wanted to catch up on every episode, you’d need to take around two months off work and log solid shifts of watching Dr. Who 7 days a week. Sadly, catching up on the entire series would be impossible as, thanks to poor archival practices at the BBC prior to 1978, 97 episodes were either destroyed, suffered from severe deterioration, or totally erased (so that the tapes could be reused).

What is unique about the missing episodes, compared to other television series “lost” by the BBC during that time period, is that devoted fans recorded the episodes in audio format. Although the video footage may be missing for those ninety odd episodes, it is possible to listen to all of them. Further, ongoing efforts by dedicated fans have slowly turned up old tapes from far-flung broadcast locations around the world.

Other notable long-running sci-fi series include Star Trek (741 episodes across 6 television series and 1 animated series), Stargate (380 episodes across 3 television series and 1 animated series), Smallville (217 episodes), and The X-Files (218 episodes).