Welp, that's a first. I have never had something jump my Hugo rankings from last-thing-read to #1 before.
LaValle has an exquisite sense of pacing and, while I would describe the writing as good without being showy for the first third, the story and style suddenly explode and you get the sheer genius of the earlier understatement. LaValle's writing is sometimes gut-wrenching in the way that makes you both afraid to keep reading and equally terrified to stop.
I hope this wins.