Favourite story series
For the purposes of this question, ‘story series’ is defined as any set of more than one story set in the same universe and revolving around the same characters; including, but not limited to, a story with a single sequel.
Given the above definition, my answer to this question is a no brainer. My favourite story series is Descent into Darkness by Athey and the sequel Harry Potter and the Breeding Darkness, adopted from Athey by Danyealle-sama.
I am also a fan of Web of Lies and Web of Lies: Revelations by Star Polaris; the Amulet of Time series by Luna Moonmonster (6 stories); the Band of Gypsies series by Natasha Vloyski (6 stories); Recnac Transfaerso and Recnac Transfaerso Alternate Ending by Celebony (2 stories if read the way I prefer, but the author has a bunch of stories in the series depending how you prefer to read it); and the Lark series by Nimori (3 stories).
There are, of course, a bunch of other series’ that I enjoy enough to re-read periodically. These, however, are the ones I enjoy the most.