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.
Love love Hitch Hikers Guide, I always go back to it when I’m feeling nostalgic for my first funny and absurd adventure, Sideway Stories from Wayside School - Louis Sachar.