I would have to say the cultures--yes, plural--from To Be Undone. Each culture is very different and very connected to its place in the world. It is interesting to play with the rigidity and beliefs of each society that Phayn, the heroine, encounters.
14. How do you map out locations, if needed? Do you have any to show us?
The overall form of the world is dictated by the story, ie what I need to happen for the story to develop as I see it. Frex, in The Traveler's Daughter, I wanted two continents connected by an isthmus to show the relation between the lands and inhabitants. Twin goddesses are the respective creators and patrons of these lands, yet they are still different. In To Be Undone, I took the idea of story informing world even further and there are geological differences that reflect a caste system. In Witherwilds, I had to plan two geographical features to incorporate for the plot to make sense.
So, yeah, plot is a big deal for me when designing a world. I plan on mapping out some floor plans, cities, etc. (something I don't normally do), but haven't done it yet.
To see the maps that I have, you can check out the links in my Day 12 post.