The three most important are carbon, hydrogen and oxygen. Without those elements, you can't make paper to write your book on. 😌
Characters that sit in that grey area. Any character who is 100% good or 100% bad makes for terrible reading.
I think different readers enjoy different things in a book. Reasons for why a person may like a book may vary from individual to individual. For me, the most important reason for liking a book would depend on how real the characters feel to me and how much emotionally involved I feel with the characters. If a book fails to make me care for the characters, I lose interest. It's even better if the book has philosophical undertones. Additionally, prose also matters. So, I can't give an opinion on what makes a good book; I can only speak for what I like. And if I fail to get interested in a book beyond ~25-30% mark, I DNF because life is short and I would rather read for pleasure. For me world-building is not important, however someone who loves world-building over everything else might love a book which someone like me may not like at all. So books of different styles are great for different people who would enjoy them.
Consistency in character.
Absolutely! What book did you like that had good characters?
I’m currently reading The Moor’s Last Sigh by Rushdie. His narrators tend to be wacky and his characters often do unconventional things, but when you read these books, you never feel like someone did something out of character. You read something outrageous and still feel like the character would definitely have done or said that.
Good prose
I also find the form of the writing to be very important! Can you give an example of particularly good prose?
Prose, themes, character arcs, plot are the essential building blocks of a novel.
All 4 of these are important for a book to be a masterpiece, with plot being the least important.
Carbon
The discussion of fiction books! Please tag spoilers and follow instance rules.