A favourite is something you can't do without, a book to which you return again and again. Mine is War & Peace, as I said. That's easy. The Heart of Darkness and The Sun Also Rises had the greatest impact on my impressionable young mind. Pride and Prejudice is a perfect gem of a book, and I read it often, but it doesn't stir the heart in quite the same way. But after that, I have to break it down much more precisely.
Let's start with historical fiction (although I have to admit my tastes are quite specific).
Most likely to leave you wondering: Alias Grace
Most likely to make you gasp out loud: Fingersmith
Most romantically bookish: Possession
Most intriguing lead character: Restoration
Most strangely compelling: Perfume
Best Napoleonic fiction: The Passion
Greatest nautical adventure: Ooh, that's a big call, I think it's a dead heat between Hornblower and Aubrey.
Most melodramatic swashbuckler: The Sea Hawk
Most gripping World War One novel: Regeneration trilogy
Most fascinating discussion of ethics: The Dream of Scipio
Best ragtime book: Ragtime
Children's books? (The books that I read when I was young, not those I read now for professional purposes.)
Most heart-thumping pirate adventure: Treasure Island
Scariest shark attack: The Coral Island
Most brilliant swordfighting: Ronald Welch's Carey series
Most likely to turn a child into a writer: Emily of New Moon (Anne of Green Gables' far more interesting cousin)
Most likely to turn a child into a recluse: My Side of the Mountain
Most likely to turn a child into an archaeologist: The Eagle of the Ninth
But I have to admit I don't read a vast amount of fiction nowadays, because it is so often disappointing. So I won't even start on non-fiction today, except for a few ideas about travel writing:
Book that most makes you want to ride into the mountains on a donkey: The Road to Oxiana or more recently An Unexpected Light
Book that most makes you want to live in Venice and never come home: Jan Morris's Venice
Greatest letters by a camel-riding Edwardian Englishwoman: Gertrude Bell
Book that most makes you want to climb mountains: Feeding the Rat
Book that makes you want to never climb mountains: Touching the Void
We could start on most over-rated travel writer or most boring camel journey that nobody ever wanted to read about, but let's save that for another day.
Feel free to leave your own categories and winners in the comments.