Ok, I'm only going to list fiction books I re-read since I mostly read nonfiction and like way too many of them to start listing.
The Patrick O'Brian Aubrey-Maturin series. Simply brilliant, shouldn't even need an introduction. Also, on a related note, check out the movie (
Master and Commander: Far Side of the World) if you haven't already. It's a beautiful piece of work.
The Sound and the Fury -- William Faulkner. Had to read this for a class. This may be a hit-and-miss suggestion; I was the only person in my class who liked it at all, but I absolutely adored it more than any other book I had to read all throughout school. Probably the difference was that I had a copy with plenty of supplemental material (I got the Norton Critical Edition, second edition) that helped me understand the context and themes and background, and had interviews with the writer, critical essays, etc etc. Definitely one of those books you read over and over and find something new each time.
Another school book,
The Stranger by Albert Camus (I can't read French so I had the Matthew Ward translation). It wouldn't be an exaggeration to say this book changed my life.
Pretty sure someone already mentioned
Paradise Lost by John Milton, so I second. As someone who dislikes both poetry and religion, I really loved this one.
Jonathan Strange & Mr Norrell by Susanna Clarke. On the lengthy side but so, so,
so worth it. Beautifully rendered, intelligently written, character-driven story of dry-witted, stiff-upper-lipped English Regency-era magicians! C'mon, what's not to love?
Finally,
The Mists of Avalon by one Marion Zimmer Bradley. Another doorstopper but such an awesome read. Helped inspire my tattoo so I have what you may call a bit of a soft spot for this one

I saw the miniseries; it was ok but it only touched upon like 10% of the book so... yeah.