Best Series is one of the newer Hugo Awards, set up to commemorate the long-running series in the field. There are some fairy strict requirements on it--such as, a previous winner may not be nominated again; and a previous nominee can only requalify if two years have passed and at least 240,000 more words have been published. (Which leads to some reservations about my top pick, as we shall see.)
Needless to say, this category adds a LOT of reading for a voter each year. I lucked out a bit this year, as three of the nominees were on my ballot, meaning I'd already completed them.
Luna, starting with Luna: New Moon by Ian McDonald
The Wormwood Trilogy, starting with Rosewater, by Tade Thompson
This actually could be expanded further, as a Sorry HELL no. I tried my best to get into both these series. I attempted to read Luna: New Moon twice and bounced off it hard both times--both the characters and the setting turned me off--and finally gave up on Rosewater after 260 pages, when I knew I didn't care about any of the people found within.
In any event, I really disliked both books. So on my final ballot, they are at the bottom, with "No Award" above.
We Have a Winnah!
There aren't any "knocking on the doors" in this category--the remaining four entries are solid finalists, and I shuffled them around a few times before deciding on the final rankings.
Planetfall, Emma Newman
This series isn't so much a sequence of plotlines as one big, world-defining event at its center and its consequences, explored through the eyes of several different characters. The first book is dragged down by its awful ending, but subsequent books are better, with Before Mars my personal favorite.
InCryptid, Seanan McGuire
Seanan regularly (at least so far) rotates appearances in this category, between this series and the October Daye series. I'm sure she'll get this trophy one of these years, but I just couldn't put her above the remaining two finalists.
The Winternight Trilogy, Katherine Arden
I nominated this. It's a lovely story based on Russian folklore, enchantingly told. One of the best things about it, for me, was watching the writer's development from book to book. When she reaches the third book, she is fully in control of her story, and the results are magic.
The Expanse, James S.A. Corey
I'm a wee bit put out with the voters for putting this on the ballot this year, only because the final book in the series is coming out next year (I believe) and unless it doesn't win and that final book isn't 240,000 words long, it can't appear again. (Or they write another novella after the final book, I suppose.) Having said that, this big, sprawling, galaxy-spanning space opera series is what this category was made for, and I had to put it at the top.
Next up: Best Related Work