Obviously, for me, Tolkien tops the list. I reread the entire trilogy every couple of years, and include
The Hobbit probably every other time. The entire collection together is my all-time favorite literary work.
In second place would be the only other novel I make an effort to reread regularly,
Centennial. I like everything I've read from Michener - I'm still not through the entire library - but this one stands out as his best to me. Even the first chapter - 40 pages on the geological formation of Colorado - holds me enraptured.
Other books that immediately stand out, in no particular order:
Brave New World - In the argument over Huxley and Orwell, I've always sided with Huxley. His dystopia is scarier because it is more realistic. As Neil Postman said: "What Orwell feared were those who would ban books. What Huxley feared was that there would be no reason to ban a book, for there would be no one who wanted to read one."
An Instance of the Fingerpost - Iain Pears' masterpiece is a case study in the use of the unreliable narrator. Four people tell the same story, except not only do they disagree on the details, they disagree on what the story even is.
Trinity - I've read several Uris novels. They are all technically excellent, but
Trinity was the only one that didn't make me feel that I was kind of, sort of, just maybe, getting lectured at a little bit.
Run - The first novel Ann Patchett wrote after becoming famous, the characters inhabit a world we lived in very recently. It's not a political novel, but the issues of race and community raised in it very much come from the optimistic vision of the possible America we all saw in the wake of the 2004 DNC (the novel includes a scene in which a character walking on campus notices an "Obama 2012" sign in a dorm window).
The Dispossessed - Probably my favorite science fiction novel. I picked it up for fifty cents at a garage sale, and sat on it for years before actually reading it.
A History of the World in 10 1/2 Chapters - Every chapter in this novel is a self-contained story, but they are all tied together beautifully.
Hit Man - One of the funnest crime novels I've picked up, it introduces Keller, your boring, every day, professional assassin and stamp collector.
The Terminal Experiment - Not strictly speaking the best novel ever written, but still one of my favorites, because there is so much going on. The premise is a doctor who wants to pinpoint the exact moment of death after a horrifying experience at an organ harvest, but in the process also accidentally discovers the human soul. Sawyer tackles the thorny issues this raises head-on, and weaves in his protagonist's personal issues into a suspenseful murder mystery involving artificial intelligence. The "twist" resolution of the mystery is a bit obvious, but the journey there is so fun, it's worth it.