For me, it's a tie between the world created in the Spellmonger Series by Terry Mancour, and the Cosmere by Brandon Sanderson. It's a tough choice too, since I love different things about the different universes they have created. I love how Spellmonger handles divinity, for example - being faith based constructs of humanity and unique to the magic of humans, a concept I adapted to my own setting. At the same time, I love how creative the Cosmere can be, particularly Roshar. There's a lot of concepts that just inspire me.