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.
It’s a toss-up between two perennial favorites;
Neal Stephenson’s “Cryptonomicon”@1999, Avon Books and Iain M. Bank’s “The Algebraist”@2004, Orbit. Both of these excel in character development, compelling narrative, adventure, and world building. Over the decades I believe I’ve read each 4 times.
Honorable mention: Ray Bradbury’s “Something Wicked This Way Comes”@1962 at least 4 times but pre-millennial. Also, a much shorter read.