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.
The Alchemist or Catcher in the Rye. I like how my perspective of the characters change as I get older in the Catcher, and I always pick up on something different when I read the Alchemist.