It is hard to choose just one, so here are my favorites:

For just wonderful word play and children's books that aren't really for children: Norman Juster's Phantom Tollbooth, Lewis Carroll's Through the Looking-glass, and The Book of Merlin by T.H. White

Great characters, including the cities that the stories take place in: Armisted Maupin's Tales of the City, A.S. Byatt's Possession, Mary Renault's Last of the Wine and Gone to Soldiers by Marge Piercy.

Fascinating World Building: The Sparrow by Mary Doria Russell, Ammonite by Nicola Griffith, Parable of the Sower by Octavio Butler.

Excellent Southern US writing: Pat Conroy's Prince of Tides, Fanny Flagg's Fried Green Tomatoes, Bean Trees by Barbara Kingsolver and Walking Across Egypt by Clyde Edgerton