What novel has the best world building you've seen?

Forty Millenniums of Cultivation
Any of Zero Destiny novels. 

Mushoku Tensei
Kono Sekai ga Game da to, Ore dake ga Shitte Iru ; The backstory of all the bugs / exploits is so hilarious. I'm sad it's not translated faster because that's one of the funniest novel I've read. If the translator reads me keep it up !  

Reverend Insanity's world building is my favorite 

hmm
Youjo Senki 

Emperor's Domination is the definition of cliche but the world building is one of the best out there. 

By far the best world building I've ever come across (in any language) is from Yue Guan's historical novels; with "Jinyi Yexing" (锦衣夜行) as perhaps the best example.  These books just do a ton in the way of explaining so many aspects of life and times during the eras they depicted.  It can be as broad in subject as culture, geography, language, peoples, and governments and as specific details about particular laws, how the people react to these laws, how they are enforced, and why they were created in the first place.  The amount of detail found within is just so enormous that there's no way any writer can invent fictional settings that come close.  Hell, I've never seen anything close in English-language historical novels.