A few days ago, I'd have said A Special Speck of Dust, but it got a lot of comments in the final days of the year. (It's also my second-most bookmarked fic of 2016, right after the tentacle porn!)

Now, I suppose it may be Say My (True) Name, a fic with name magic for a song fandom, which I expected to get a few more comments. Everything else, I understand why it's where it is (small fandom/kinky content/tailored to recip likes rather than what's fandom popular), but I expected this to have another few commenters.