Ooh, this is a wonderful topic!
My favorite genres are the fantasy/sci-fi duo. You might not be able to tell it from my indecently short, brief posts around here, but I would actually consider myself as much of a writer as an engineer. Goodness only knows how much stuff I've written in my day, and most of it is fantasy or science fiction. I prefer these genres because their trademark characteristic is a wider realm of possibility, as physical and scientific limitations are easily circumvented in the name of the imagination. Thus, despite their tendency to be stereotyped as space battles and cloak 'n dagger tales, science fiction and fantasy in my opinion are the broadest of all literary genres.
I have several favorites:
The Chronicles of Amber, by Roger Zelazny
This was the first adult fantasy series I ever read, courtesy of my dad, who got my hooked on sci-fi and fantasy long ago. As a result it remains dear to my heart, but even in modern times I consider it one of the better fantasy works I've ever read, especially the first four books out of the series of ten.
The Silmarillion, by J.R.R. Tolkien
As much as I liked the lush detail of The Lord of the Rings, the sweeping epic of The Silmarillion is much more powerful to me. In one short book, Tolkien manages to touch on so many of humanity's great myths. The Silmarillion is like a rough sketch of the history of everything, and thus it fires the imagination like nothing else I've ever read. Tolkien also had a writing style more mature than any other fantasy author I've ever read.
Battlefield Earth, by L. Ron Hubbard
The book, not the movie. I read it when I was a kid, and for a number of years it was my favorite book. I don't remember much about it anymore, but it greatly inspired my early teenage imagination, which in turn led to some of my best writing from that era.
The Door into Summer, by Robert Heinlein
This is one of my dad's favorite books, and I like it a pretty well myself. Heinlein doesn't always sit terribly well with me for his politics, but you can't deny the man was exceedingly talented.
From the Mixed Up Files of Mrs. Basil E. Frankweiler, E.L. Konigsburg
Why did I put a children's book on my list? Well, I happened to really enjoy it when I was a child! (Right along with The Girl Who Owned a City and Lisa: Bright and Dark!) I may not be able to enjoy it now as much as I did back then, but it helped shape my entire literary style (as did the other two). To this day whenever I come across a sentence with the words "off on" in succession, I think of this book and the protagonist's haughty grammar lessons to her little brother.
Beowulf
Of all the books I read in high school, strangely I liked this one the most, along with such other classics as the poetry of William Blake, Milton's Paradise Lost and Spencer's Faerie Queene, complete with the unforgettable Britomartis! I think you can see what all of these different works and authors have in common that would appeal to me so well. Or is it not so obvious for someone who isn't me? I'm curious.
Táin Bó Cúailnge
Oh my goodness...you'll simply have to read this book to believe it. Ireland's greatest epic, the Táin (rhymes with "coin") is simultaneously one of the most bombastic and ridiculous stories I've ever read, yet so earnestly executed and passionate, and that's why I love it. And when Queen Maev is slightly bested by her husband Ailill's possession of the phenomenally fertile bull Finnbhennach, who had been born into Maev's herd but scorned being owned by a woman and decided to transfer himself to Ailill's, she determines to get the equally potent Donn Cuailnge to balance the books. After unsuccessfully attempting to borrow the bull, she raises an army, including Ulster exiles led by Fergus mac Róich and other allies, and sets out to capture thus studly creature. And, well...you'll just have to read it for yourself. What a story, and all for one bull!
Of course I have many preferred books across most genres, from universally acclaimed stories like Shakespeare's Hamlet, to llittle-known masterpieces like Borges' Labyrinths...from Uris' Trinity to Michener's The Floating World. But I would be remiss if I did not mention my favorite book of all:
Cosmos, by Carl Sagan
Like chess, it's a book that you can read in a day, but may learn from for a lifetime. It certainly did a number on me. Based on Sagan's award-winning documentary of the same name, Cosmos is an exposition of science written for people who don't wear white lab coats and never intend to. It's easy to digest, and deeply thought-provoking on the same "everyday" sort of level that made Jesus' simple parables so accessible. Read this book if you ever get the chance. In fact...go make the chance.
Delightful thread!