Fantasy :
George R R Martin - "A Song of Ice and Fire" - four books in this series so far. Very political, "Wars of the Roses" type fantasy but hugely engrossing. Political Skulduggery, backstabbing kings and courtiers, Noble dynasties warring openly and behind closed doors, fantastic creatures stirring and portents in the sky... wonderful stuff.
LOTR as Asguard said. Life wouldnt be the same without it.
Sci-Fi :
Orson Scott Card - "Enders Game" et all (total of five books with more on the way)
VERY philosophical sci-fi, great if you like sci-fi thats not too hardcore and very readable.
Frank Herbert - "Dune" series. Goes without saying. Forget the movie.
General Fiction :
ANYTHING by Louis De Bernieres.
Great satirical, black humour centred mostly around a fictional South American Nation and its inhabitants from all walks of life. The only book this guy has written I'm not overly enthused about is "Captain Correlli's Mandolin" however, any movie buff will have seen it... trust Hollywood to pick the worst book written by this author and turn it into a schmaltzy movie. The book had FAR more depth. I'm probably being unfair to Correlli, but in the light of his other stuff I thought it was not as good. Read them all anyway.. they're all top shelf.
Horror :
Anne Rice - "The Vampire Chronicals" and the Mayfair witches books. Pure magic, the woman's a genius.
I've also got a soft spot for Poppy Z Brite.
Political Commentary (among other things) :
Pretty much anything by P. J. O'Rourke or Hunter S Thompson. these guys have an understanding of the American psyche and know how to write about it.
Classics :
Ernest Hemingway - Almost anything, But I think my favourites are "The Old man And The Sea", "For Whom the Bell Tolls" and "Islands In The Stream".
John Steinbeck - ANYTHING. Best of his in my opinion -
"The Grapes Of Wrath", "Of Mice And Men" but anything of his is fine.
Jack Kerouc - "On The Road"
Fantastic book about the Beat Generation, started a new trend in American literature for "Stream of Conciousness" writing. A must read.
Ayn Rand - "The Fountainhead", "Atlass Shrugged"
A completely different way of looking at the capitalist way of life, a philosopher in her own right. Essential reading for those who have turned to the left wing in search of answers to todays "version" of capitalistic society. And, anyone else for that matter. Dont complain to me if you cant stomach it, unless youve read the whole book and have valid arguments as to why she's wrong.
That lot's just my favourites... more where they came from.