Nominated by Namohysip, Kindoflame and Venia Silente
If there's one thing you can say Fledglings dedicates way too many words to, it's the setting. No seriously, I… I think it's almost a full third of the fic.
It's hard to find competition for this one when it comes to worldbuilding. The world and the story for it are well developed and there's always some wiggle room and hooks available for showing more.