"Best of" book lists are always popular, but they are answering the wrong question.
I have read thousands of books over my life. And written a few.
I started with science fiction such as Wasp, Foundation, Dune, Avram Davidson, City, Slan, the science fiction books club collections (Galaxy Readers), you name it.
That was pretty much it until I reached college. I branched a little ways out to Herman Hesse and Kurt Vonnegut, but really, stayed right in my comfort zone of SF.
The breakout for me was when I read Gravity's Rainbow. I kept mumbling to myself: "can a writer do that?" I struggled with the characters and the subplots.
When I finished, my mind had been stretched. And it stayed stretched. I resolved to read outside of SF no matter how hard or how boring. I compiled best of lists of all kinds of literature. I collected university course lists in literature from places like Cal Tech, Berkeley, Yale, Harvard, MIT. BBC, Times Literary Supplement lists.
I worked through them all. It didn't take too many years. I filled notebooks with thoughts that arose as I read. Then I was ready to start writing.
I still enjoy my SF even though I understand its limitations; now I can enjoy comparing that experience to the experience of reading Maya Angelou or Mary Gaitskill. I can enjoy the quality writing in the New Yorker. I enjoy the reviews I used to find opaque in the Times Literary Supplement. And I can participate in looking at life around me and figuring out how to express it, even though never expressed before. And I know when an expression is likely fresh and because I have never read it before.
So my advice is--rather than looking for the magic set of books that is like a literary vitamin supplement, instead constantly choose books outside of your comfort zone. Yes, don't forget to reread your favorites lest you begin to fear difficulty, but try to understand why other educated people consider books great that you do not understand now. My current reading list is Wolf Hall, Marquez, The Hydrogen Sonata, Ian Fleming, Gone Girl, Churchill's History, Redwall, Murakami, Eco, incest porn, Nabokov, Joyce... in other words, the gamut of human literature in English.
As Tom Wolfe said "Let's not mince words: literary lists are basically an obscenity. Literature is the realm of the ineffable and the unquantifiable; lists are the realm of menus and laundry and rotisserie baseball. There's something unseemly and promiscuous about all those letters and numbers jumbled together. Take it from me, a critic who has committed this particular sin many times over."
So, the best book to read next s the one just an epsilon out of your current reach. One that takes a little struggle to release all its pleasure.