Detailed Avatar Worldbuilding
The world of Avatar, even if it's a movie not a book, has IMO awe inspiring worldbuilding. The detail in how the planet works, what's it made of, size, even what it orbits, to the ecosystem, is almost perfect. I found very few holes, only once do I remember asking "But what about?"
I found today at this link, that they followed the attention to detail into the Na'vi language. It is a full language, like Klingon. The article promises that info on the internet on how to learn/speak it will be posted soon. Cool.