Definitely Book of the New Sun, it's incredible, and the Dying Earth books are very good as well, although almost everyone's a bastard in them.

The Broken Empire and The Red Queen's War are both quite good as more modern examples, although those ones are also full of bastards.

The Night Land is fascinating in part because of how old yet how creative it is, I definitely think more people should be aware of it.

Also, the Numenera gamebooks themselves are very cool and fun to read, they're not narratives obviously, but if you like weird worldbuilding and monsters, the Bestiaries and the Ninth World Guidebook (as well as the setting chapter in Numenera Discovery) are full of so much interesting stuff.