Pornokitsch

Susanna Clarke (of Jonathan Strange & Mr. Norrell) adds to the Guardian's list of must-read novels with a half dozen of the best 'imagined worlds (article spotted on the exceptional Middle-Earth Blog)':

CS Lewis: The Chronicles of Narnia

JRR Tolkien: The Hobbit; The Lord of the Rings

Philip Pullman: His Dark Materials

Terry Pratchett: The Discworld series

Ursula K Le Guin: The Earthsea series

This is, essentially, the Child's Bumper Book of High Fantasy. Epic, plot-driven series with some of the best-defined world-building in this (or any) genre. I've heard Clarke speak on a few occasions, and despite the sedate, character-focused plot of Jonathan Strange, she is always very open about the fact that her real joy is in world building (as evidenced by the book's copious footnotes).

Personally, the Discworld series is my least favorite of all of these. Xanth-like, it has stretched out far too long, with the jokes becoming increasing stretched. (Brief pause as 16 million British fantasy fans climb through the Internet and beat me to death with copies of their Science of Discworld audiobooks) However, as an example of world-building, Discworld is phenomenal - with Pratchett gleefully detailing every bizarre nook and cranny.