I voted Kamigawa, but I want to specify the new Kamigawa set. Magitech cyberpunk.
Close in contention would be Eldraine, Capenna, and a book about many planes from the posthumous perspective of Gideon, with notes by various members of the Gatewatch, and other planeswalkers close to him.
Or maybe from the perspective of the pre-War of The Spark Gatewatch.
I might settle for a post-whatever is happening right now perspective, too, perhaps. Especially if it ends with Weatherlight being able to go between planes again. Great reason for a coherent guide to the planes.