My vote goes to: Blackfish City by Sam J. Miller. A sexy and urbane thriller set in the aftermath of climate catastrophe, which deftly handles multiple viewpoints, including technologically mediated shared human-animal consciousnesses. I would say that it was the best novel that I read of any genre that was published in 2018, but I fear that, because my reading list over the last year has been lighter than usual on contemporary fiction, that might seem like faint praise. Let me say then that it is arguably one of the best novels of this decade.
The Black God's Drums by Phenderson Djèlí Clark wins my vote through having the most original setting of this year's nominees.
I will never not put this category name in scare quotes, as this term that as far as I can tell is current only among science fiction and fantasy fandoms too often ends up encompassing two disparate literary phenomena: long-ish short stories, sometimes overly long; and brief novellas, sometimes too brief to be fully realized. I have read and enjoyed pieces that fit the boundaries of this category, but none of this year's nominees have won my vote. (To be fair, for some of them it was only because interlibrary loan has been too slow for me to receive and read them before the deadline for ballot submission.) Abstention / no preference.
Phenderson Djèlí Clark wins my vote again, this time with a story that tests the limits of fantasy at the levels of concept, narrative, and stylistics, "The Secret Lives of the Nine Negro Teeth of George Washington." While I am disappointed at the entirely anglophone nature of the nominee list in a year when a number of brilliant stories in translation found their way into science fiction and fantasy publications, this story is head and shoulders above almost anything else the genre brought us in 2018.
Abstention / No Preference for the Bradbury Award (dramatic presentation--my media consumption habits are dominated by print), Norton (YA; as it is--too much of what was nominated for the "adult" Nebulas reads like YA for my taste); or Game Writing (also not my bag). I would be tempted to vote for Black Panther for the Bradbury, as I did actually see it in the theater, but to do so without having yet seen Sorry to Bother You seems unfair to the latter.