In no particular order:
Winnie-the-Pooh (a beautiful elegy to childhood);
The Wind in the Willows (a charming evocation of the British Edwardian countryside);
His Dark Materials Trilogy (Stunning, dark, challenging and ultimately, exultantly humanist);
The Hobbit (The beginning of a new mythology from the man who taught mythology at Oxford);
Artemis Fowl (a cheeky and irreverant poke at traditional fantasy stories).