2. Perry Rhodan
This is seen as the best-known science fiction series of all time, accounting for more than 2 billion (not a typo) copies sold since its premiere in the 1960s, first creating a buzz in Germany, then being translated worldwide, but not ultimately succeeding in America in English.
It was originally published in German alone, but was translated into other languages over time. This is where the incredible publication and sales numbers come from. In Germany it’s a weekly novella in a magazine.
On the surface, this series has science fiction concepts: hyperspatial
translation and positronic brains — concepts that Isaac Asimov created in his stories. The M13 cluster also exists. Everything else about this series (functional immortality — never aging, but violent death possible, multiple time-lines/realities, the psionic web, moralic code, many other fanciful ideas) is a made-up concept.
Sci-Fi legend Forrest J. Ackerman championed English translation of this series. He said that serious German SF fandom said how it hated this series, yet it was still a top seller in the country. It proved the same case with the English translations by Ace Books, which ceased publication with 117-118 in the early 1980s (it’s over 2400 stories-long). Then-head of Ace Tom Doherty found them to be too juvenile in quality.
Good or bad, these books (and spin-off series) inspired music, film (George Lucas cites it as one of his influences), and is even an inspiration to other science fiction series. Would that other series could go so long!