Appearing this year for the first time as an official category, we have Best Series:

A multi-installment science fiction or fantasy story, unified by elements such as plot, characters, setting, and presentation, appearing in at least three (3) installments consisting in total of at least 240,000 words by the close of 2017, at least one (1) installment of which was published in 2017.

Last year, Worldcon 75 had a special Best Series category to give it a test run before it was ratified as one of the ongoing categories.  Back in 1966, a Best All-Time Series Hugo was awarded to the Foundation series by Isaac Asimov.

This category has a couple of additional caveats.  A previous winner may not be a finalist in this category again.  Any other finalists must have at least two additional installments (with at least one published during the qualifying year) totalling at least 240,000 words since their last appearance in order to be eligible again.  I think it’s best to assume that last year’s Best Series winner and finalists are therefore ineligible even though that was technically a different one-off special category.  Of course, that will be up to the Hugo committee should any of them get enough nominations.

I’m woefully behind in reading series, and there’s no time to catch up or start something new now.  After looking through the massive list of possibilities compiled by the folks at File770, I found only seven series that I know I’ve read any part of (disregarding last year’s finalists).  And for various reasons, I don’t think I’ll be nominating any of them.

Broken Earth by N. K. Jemisin, The Stone Sky
Jemisin herself has said that she’d rather this wasn’t nominated.  And yeah…a trilogy which has already received two Best Novel Hugos (and may well be a finalist again) doesn’t really need to be acknowledged as a series as well.  I see this category as intended more for honoring situations where the installments are good, but the overall series is where it really becomes Hugo-worthy.  Not that I’d want those that had won other Hugos to be explicitly excluded.  It would save me some reading this summer if it were a finalist.  Although she might decline the nomination anyway.

Imperial Radch by Ann Leckie, Provenance
I’m almost finished reading Provenance.  While there’s no question that this occurs in the same universe and shortly after her previous trilogy, it’s really only very tangentially related.

Stormlight Archive by Brandon Sanderson, Oathbringer
I enjoyed the first two, but I haven’t started the latest tome yet.  And since this is projected to be a ten book series, I think nominating it now is a little premature.

A Song of Ice and Fire by George R. R. Martin, “The Sons of the Dragon”
Um, yeah…no.  And also, Martin himself has requested it not be nominated.  I’ve loved this series for 20 years, but it’s a mass of cliffhangers right now.  I think anything nominated for series needs to have reached some conclusion, if not the overall ending.

Wild Cards by George R. R. Martin (and many others), Mississippi Roll
While I don’t feel it necessary to have read all of this series to nominate it, I’ve probably only read a handful of the stories and none of the actual collections or mosaic novels.

World of the Five Gods by Lois McMaster Bujold, Prisoner of Limnos
I’ve only read the first two novellas in the Penric and Desdemona sub-series and none of the main series.  Going by the word counts given over at Rocket Stack Rank, the sub-series of novellas hasn’t quite reached the 240,000 word requirement to be nominated on its own.

Xuya Universe by Aliette de Bodard, “First Presentation”
This is a series of 24 short fiction works, and the author has confirmed that it meets the word requirement. But I’ve probably read less than a handful.

All of that to say, I’ve got nothing here!  Which series do you plan to nominate?