Really, not possible to have a single favorite. Maybe favorites by genre?

Fiction: Shadow of the Wind by Carlos Ruiz Zafon

https://en.m.wikipedia.org/wiki/The_Shadow_of_the_Wind

Science Fiction: Little Fuzzy by H. Beam Piper (bonus! public domain!)

https://en.m.wikipedia.org/wiki/Little_Fuzzy

Project Gutenberg: https://www.gutenberg.org/ebooks/18137

Fantasy: Lord of the Rings

https://en.m.wikipedia.org/wiki/The_Lord_of_the_Rings

Horror:

Books of Blood by Clive Barker

https://en.m.wikipedia.org/wiki/Books_of_Blood

Of particular note are the stories “The Yattering and Jack” and “The Body Politic”.

Western:

The Dark Tower series by Stephen King

https://en.m.wikipedia.org/wiki/The_Dark_Tower_(series)

Romance:

Griffin and Sabine series by Nick Bantock

https://en.m.wikipedia.org/wiki/Griffin_and_Sabine

Also - Outlander by Diana Gabaldon

https://en.m.wikipedia.org/wiki/Outlander_(book_series)

Travel:

Into the Heart of Borneo by Redmond O’Hanlon

https://www.penguinrandomhouse.com/books/123474/into-the-heart-of-borneo-by-redmond-ohanlon/

Also - In Trouble Again by Redmond O’Hanlon

https://www.penguinrandomhouse.com/books/123475/in-trouble-again-by-redmond-ohanlon/

Religion:

Rescuing the Bible From Fundamentalism by Archbishop John Shelby Spong

https://www.harpercollins.com/products/rescuing-the-bible-from-fundamentalism-john-shelby-spongjohn-shelby-spong?variant=41174963781666

Also - Liberating the Gospels: Reading the Bible with Jewish Eyes by Archbishop John Shelby Spong

https://www.harpercollins.com/products/liberating-the-gospels-john-shelby-spong?variant=41245649600546

Mythology:

Tales series by Chronicle Books:

https://www.chroniclebooks.com/collections/tales