Not including short story collections, poetry collections, anthologies, etc.
For me, I believe it’s Oroonoko by Aphra Behn because it was on the syllabus for three different classes that I took in college. Other than that, the most I’ve read a single book is twice, and that was Telegraph Avenue by Michael Chabon. I’ve read Dracula 1.5 times. Didn’t finish it the second time around.
I’m a high school teacher, so I often read books aloud to my students. The book I’ve read the most is The Secret Life of Bees, though The Great Gatsby will soon catch up to it and surpass it.