I can't resist making a list. But I cheated and used some of my non-fiction allotment for fiction. I didn't go over 20 total, though. My criterion was mostly what we used in my old book club, where we voted on each book: "pure pleasure." Though the significance of a book may have factored in to a greater or lesser extent.
(unranked)
Fiction:
1. A Prayer for Owen Meany / John Irving
2. Sometimes a Great Notion / Ken Kesey (I had to put this book down and walk around once when it got too intense.)
3. To Kill a Mockingbird / Harper Lee
4. Jane Eyre / Charlotte Bronte
5. Hamnet: a novel of the plague / Maggie O’Farrell
6. A Gentleman in Moscow / Amor Towles
7. Mrs. Caliban / Rachel Ingalls
8. The Professor's House / Willa Cather (mainly the middle section "Tom Outland's Story")
9. The Remains of the Day / Kazuo Ishiguro
10. The Magus / John Fowles
11. Atonement / Ian McEwan
12. Evening / Susan Minot
13. Rebecca / Daphne du Maurier
Non-fiction:
14. Solito / Javier Zamora
15. Confederates in the Attic / Tony Horwitz
16. The World in A Grain: The Story of Sand and How It Transformed Civilization / Vince Beiser
17. How To Change Your Mind: What the New Science of Psychedelics Teaches Us About Consciousness, Dying, Addiction, Depression, and Transcendence / Michael Pollan
18. My Stroke of Insight: A Brain Scientist's Personal Journey / Jill Bolte Taylor
19. The latest book by Michael Gazzaniga, cognitive neuroscientist (The one that blew my mind I read in the 90s, and I feel sure there have been new amazing developments since then.)
20. Metaphors We Live By / George Lakoff (linguist) & Mark Johnson