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.
If we don’t take into account short children books, then it is probably The Hobbit, read for many times to each of our children.
Of books I would choose for myself, they were probably Harry Potter, The Hitchhiker’s Guide to the Galaxy, The Chronicles of Amber, Master and Margarita.