Editors are a vital part of the whole literary business. The work they do in selecting, acquiring, nourishing, critiquing, shepherding, and making ready for publication at all levels the fiction we love to read is invaluable, as much as it is, to the reader, invisible (except when it's been poorly done, or not done at all, and then we can all see what the editor brings to the bookshelf).
In considering the Hugo finalists for best editor, short and long forms, I paid attention both to the work done by the finalists during the past year, and to public statements I could find by the finalists about their philosophies and approaches to editing and to science fiction. I consider the latter to be important because editors are gatekeepers, and their attitudes and philosophies determine what works are published, and thus shape the future of this complex genre.
Editor, Short form
This year's finalists were all well-known to me, John Joseph Adams, Neil Clarke and Ellen Datlow in particular. I don't read Asimov's regularly, but I do often enough to be familiar with Sheila Williams. Jerry Pournelle I know mostly as a writer of hard sf.
Adams is a remarkably prolific editor - in addition to editing the online magazines Lightspeed, Fantasy and Nightmare, he edited or co-edited five anthologies in 2015 (that I know of). Under his guidance, special editions of Lightspeed, Fantasy and Nightmare devoted to the work of women and queers were published, and a new round of special editions featuring the work of people of colour is in the works. I've read any of the anthologies he's edited, and many stories from those online magazines.
Neil Clarke is the editor of Clarkesworld, an online magazine publishing some of the most interesting and innovative short fiction around. Last year's offerings included two of the best short stories of the year - Naomi Kritzer's "Cat Pictures Please" and Aliette de Bodard's "Three Cups of Grief, By Starlight."
Ellen Datlow has been a name to conjure by in the workd of science fiction for a very long time. In addition to her many print anthologies - three of them released in 2015 - she is one of the editors at tor.com, another source of innovative new short fiction. Among the pieces she edited for tor.com in 2015 were the excellent novellas "The Pauper Prince and the Eucalyptus Jinn" by Usman Malik and "The Waters of Versailles" by Kelly Robson, and the wonderful novelette "Fabulous Beasts" by Priya Sharma.
Sheila Williams is the editor of Asimov's Science Fiction, which has long been one of the giants in the field. In 2015, works published under her aegis included Eugene Fischer's compelling novella "The New Mother," and Sarah Pinsker's "Our Lady of the Open Road."
Jerry Pournelle is a noted science fiction writer. He also edited a fair number of anthologies back in the 80s and 90s. However, the work he edited in 2015, There Will Be War X, appears to be his first sf anthology in over 20 years. And my impression after skimming it was that it was uneven - some very good work, some average work, some work that coukd have been stronger, clearer, or better written. Not an impressive body of work from 2015 on which to hang a Hugo, I'm afraid.
Editor, Long form
Sheila E. Gilbert, Liz Gorinsky and Jim Minz provided the Hugo Voters Packet with lists of novels they had edited in 2015 in support of their finalist status. The other two finalists did not, and I was rather disinclined to hunt for further information in either case.
Sheila Gilbert is co-publisher of DAW Books. Her list of 2015 works included Michelle West's Oracle and Tanya Huff's An Ancient Peace - both works from writers I consider to be among my favourite "must buy" authors.
Liz Gorinsky is an editor at Tor Books / Tom Doherty Associates. Her list included such notable works from 2015 as Liu Cixin's The Dark Forest, Catherynne Valente's Radience, and Mary Robinette Kowal's Of Noble Family.
Jim Minz is an editor at Baen Books. I was not familiar with any of the books mentions on his list, but a quick check of reviews indicated that while the books he worked on were not to my taste, his editorial hand was involved in several popular and well-received novels.
Toni Weisskopf is both editor and publisher at Baen Books. She may well have edited some remarkable books in 2015, but as she didn't care to let the Hugo voters know what they were, there's no way I can assess her editorial contributions in 2015 or consider her as a finalist in this category.
The last of the finalists also failed to provide any indication as to the works he edited in 2015, or the books put out by his small press publishing house. But that's just fine by me, as everything I have read by the man confirms my belief that his vision of science fiction is so antithetical to mine that I could never vote for him.