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.
Now I’m getting to reread it with my 1st grader! It’s amazing to reread stuff I’ve read many times, but have a new perspective on it all.
This one is great on re-reads, a lot of British humor is so subtle that it is funny as a kid but you catch more jokes as you get older.