For sheer imagination it has to be:

The Gormenghast Trilogy by Mervyn Peake

Though the Master and Margarita by Mikhail Bulgakov follows very closely.

Woman in White, and the Moonstone by Wilkie Collins were books that kept me reading till 4 am - a thing I hadn't done fro years.

Auto da Fe by Elias Canneti was great - how could you resist a book about "an evil minded chess playing dwarf of evil propensities" or such like.

Use to read Alice in Wonderland/Through the Looking Glass each year

I almost forgot - Foucault's Pendulum by Umberto Eco.

Rana