When pressed for an answer, I say that House of Leaves is my favorite book of all time.
Other top contenders include:
It, by Stephen King
Invisible Monsters, by Chuck Palahniuk
The Giver, by Lois Lowry
I, Lucifer, by Glenn Duncan
Sharp Objects, by Gillian Flynn
The Collector, by John Fowles
The Silence of the Lambs, by Thomas Harris
The Phantom Tollbooth, by Norton Juster
Perks of Being a Wallflower, by Stephen Chbosky
The Presitge, by Christopher Priest
Slaughterhouse-Five, by Kurt Vonnegut
To Kill a Mockingbird, by Harper Lee
Illusions, by Richard Bach
How to Set a Fire and Why, by Jesse Ball
Legion, by William Peter Blatty
As for favorite authors overall, I would say my list has to include Stephen King, Gillian Flynn, Neil Gaiman, Lois Lowry, Glenn Duncan, H.P. Lovecraft, Arthur Conan Doyle, and J.K. Rowling.