This question is one that I quickly settled on in large part because of the genre of books that I normally read. Most of my reading is in the SciFi/Fantasy genres (although I will read just about anything that sounds interesting) and in many of those books, the worldbuilding is in many ways just as important to the story as the characters and plot. How the magic is used in the books is vital to the overall setting, as well as everything else about the world including magical creatures, strange architecture, or even the weather in the book.
This one was a toss up between two different books, and ultimately I’m choosing Brandon Sanderson’s novel The Way of Kings, the first book in his Stormlight Archive series. (The other series that I had in mind was Alan Campbell’s Deepgate Codex, with Scar Night and it’s sequels.)
I’ve made no effort to hide the fact that I’m a huge Sanderson fan on this blog. And while all of his books have unique settings for fantasy novels, The Way of Kings goes above and beyond what he has done for the worldbuilding of his other books. In Roshar, storms as strong as hurricanes ravage the world every couple of weeks, because of this, all of the housing of people in the world has been changed to deal with the savage winds, and so has all of the plant life. All of the plants have evolved to the point that they are able to hide inside the ground to protect themselves from the storms when they come.
There is also a massive history of the different types of magic within the world. The most well known among the characters we see in the first novel is simply the armor and weapons that many of the nobles use. The shardblades and shardplates used in warfare are brutally powerful weapons and armor that allow one man to have to strength to defeat hundreds if not thousands of others during a single battle. There are also hints of other magic systems that are used throughout the book, many of which will be explained during other books in the series.
The last part of the worldbuilding that I’m going to bring up is the Spren. Spren are magical creatures that appear with nearly everything in the world. There are hundreds of different types and they are so common that people don’t even think twice when they see them. There are windspren, rotspren (around injuries), gloryspren (during time of celebration), and even deathspren. Those are just some of the ones that I could remember off the top of my head, there are many others mentioned during the course of the novel.
Along with having one of the most interesting settings you’ll see in a novel, this is also a really good book that I highly recommend.