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.
Watership Down here too. At least twenty times. I used to read it annually but hadn’t for quite a few years until quite recently. That felt like a new experience in that I really appreciated every word and even noticed a few new things. I’ll write more about it soon.