Considered but Excluded:
All the David Attenborough/BBC nature docs - as much for practicality as anything. I've seen a lot of them and a lot of them are very good, but they could almost be seen as a continuous series where they put out a new one most years with a different focus, but following the same format. Some are direct sequels (Blue Planet II), others more indirect. Also they do blur in my memory so it'd be hard to rank apart.
The Young Pope/The New Pope - it's basically now a continuing series with two seasons.
The Kingdom - effectively multiseason & they would probably have made more if possible.
Twin Peaks: The Return - I can't really consider it as a standalone. It's season 3.
ones I was borderline but did include:
Heimat II - it's a follow on, but it would also standalone, you don't need to have seen Heimat. And I love it very much so wanted it in
Watchmen - it may get followed up, but even if it does, this is likely to be a separate thing (different characters in same universe)
The Night Manager - there's talk of a season 2 but it hasn't happened yet & the first is an adaptation of a book that functions as miniseries.
Nathan Barley - it's a sitcom and I daresay it might have had further seasons if it'd been a big hit, but there is a thread of plot through it and there is just the one season so put it in.
May extend list yet, might be a few things I've forgotten/overlooked.