I bought and read all four of the books marketed as "Tor.com novellas". It was more difficulty than I expected to find a listing of authors, title and publication dates - the best I found was this prospective listing of the covers posted in June, before any of them had been issued, and although all the listed works do appear to have been published on schedule, it would have been helpful to have an index. More seriously, by my count two of the four are over the 40,000 word limit for the Hugo Best Novella category - I reckon that Sorcerer of the Wildeeps by Kai Ashante Wilson is more like 43,000 and Sunset Mantle by Alter Reiss looks like 42,000 to me. I'm open to correction, of course, and even if I'm right, both are within the range of discretion for the Hugo administrators.
I read the four on a transatlantic flight, which may not be the best circumstances to appreciate them. Paul Cornell's Witches of Lychford grabbed me; the other three (the last is Binti by Nnedi Okorafor) less so. So I'm adding Witches of Lychford to my nominations list.
While we're on separately published novellas, Subterranean Press's Speak Easy, by Catherynne M. Valente, also failed to grab me; Valente's prose is gorgeous as usual, but I was not engaged by the plot or characters. I am in some doubt as to whether my hit rate for separately published novellas justifies the outlay in acquiring them, and I am considering how thoroughly I feel I need to look at all nine of the Tor.com novellas published in the last quarter of 2015.
From the Tor.com website, the three stories that jumped out at me were "Islands off the Coast of Capitola, 1978", by David Herter, a novelette; "Fabulous Beasts", by Priya Sharma, also a novelette; and "Please Undo This Hurt", by Seth Dickinson, a short story at just over 7,000 words.
Clarkesworld achieved the impossible with an anthropomorphic robot story that actually worked for me, "Android Whores Can't Cry", by Natalia Theodoridou - in general, it's a trope that I absolutely hate, but this had sufficient originality and verve to overcome my resistance. The other two that particularly grabbed me were "Security Check", by Han Song, and "Loving Grace", by Erica L. Satifka, but neither sufficiently to get on my list. Clarkesworld also published the worst story I have read from 2015, "The Hunger Tower", by Pan Haitian, which has been expertly dissected by Greg Hullender and Vivienne Raper.
From Asimov's, the standout story for me was "Caisson" by Karl Bunker. Greg Hullender complains that it had no sf element at all, but I think he's simply wrong; it's about a fossil egg that (probably) hatches. I also liked “Acres of Perhaps” by Will Ludwigsen and “The God Year” by Jim Grimsley. One story where the sfnal element did seem to me completely irrelevant to the plot was “Calved” by Sam J. Miller. I suspect it will pick up a fair few nomination votes but mine won't be one of them.
Perhaps I was suffering from pre-Christmas slump at the time of reading, but Strange Horizons' fiction didn't grab me as much as on some previous readings. Having said that, I did enjoy "It Brought Us All Together", by Marissa Lingen, a great high school plague story.
I see that my tastes are in almost complete opposition to Greg Hullender's aggregation at Rocket Stack Rank (apart from "The Hunger Tower"), but I really appreciate the work he has done in pulling the lists of recommended stories together for July, August and September. Likewise I have precisely one crossover (Priya Sharma's "Fabulous Beasts") with the Ladybusiness list for the third quarter. Tastes vary.
What I'm still considering for nomination
Lois McMaster Bujold, Penric's Demon (Spectrum)
Paul Cornell, Witches of Lychford
Eugene Fischer, "The New Mother" (Asimov's, Apr/May 2015)
Kristine Kathryn Rusch "Inhuman Garbage" (Asimov's, Mar 2015)
Allen M. Steele, "The Long Wait" (Asimov's, Jan 2015)
Eneasz Brodski, "Red Legacy" (Asimov's, Feb 2015)
Paul Evanby, "Utrechtenaar" (1, 2 - Strange Horizons, June 2015 - surprised to find it only 9,000 words)
Sarah Pinsker, "Our Lady of the Open Road" (Asimov's, Jun 2015)
Priya Sharma, "Fabulous Beasts" (Tor.com, July 2015)
Vandana Singh, "Ambiguity Machines: An Examination" (Tor.com, Apr 2015 - at 7800 words it just scrapes into this category)
Karl Bunker, "Caisson" (Asimov's, August 2015)
Nino Cipri, "The Shape of My Name" (Tor.com, Mar 2015)
L.S. Johnson, "Vacui Magia" (Strange Horizons, Jan 2015)
Jay O'Connell, "Willing Flesh" (Asimov's, Apr/May 2015)
Robert Reed, "The Empress in Her Glory" (Clarkesworld, Apr 2015)
Kelly Robson, "The Three Resurrections of Jessica Churchill" (Clarkesworld, Feb 2015)
Iona Sharma, "Nine Thousand Hours" (Strange Horizons, April 2015)
Natalia Theodoridou, "Android Whores Can't Cry" (Clarkesworld, July 2015)
All three categories full, but I shall read the fourth quarter's sf from those outlets anyway, and a few more, then start pruning.