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.
Outside of the kids books for my kids, I’ve probably stamped my Narnia passport 7+ times. More recently, Douglas Adams is still my favorite author, and Jim Butcher’s Dresden Files are very re-readable!