Hard SF: The first and seminal work was either A Martian Odyssey by Stanley Weinbaum (1934), or Mission of Gravity by Hal Clement (1954). Stanley Weinbaum was earlier. I think the best and most influential modern example is Dragon's Egg by Dr. Robert Forward. It seemed to me he brought Hard SF back from the verge of extinction.
Social/Anthropological SF: The one who wrote it best in The Old Days was Chad Oliver For modern times, I'd nominate for prototype The Left Hand of Darkness by Ursula LeGuin.
Philosophical/Idea SF: More Than Human by Theodore Sturgeon. Or maybe Stranger in a Strange Land by Robert Heinlein -- I don't think it was actually a great book, but it was sure influential. It greatly influenced how *I* spent the 60's.
Fantasy: The Hobbit was certainly influential. I'm not sure I could nominate anyone who had a greater impact on the field of fantasy than Tolkien. There are novels that I treasured as much, that influenced me personally as much or more, that I regarded as just as good or even better as novels. The Last Unicorn by Peter Beagle. Was that reponsible for the resurgence of unicorns in fiction, or was it just the crest of the flood? The Little Country by Charles de Lint. The Empire of the East by Fred Saberhagen. And in a category all its own, Silverlock by John Myers Myers. As the book that has probably inspired more filksingers than any other single book, it has to be the head of *some* category.
Horror: Personally, I never got the excitement in Bram Stoker's Dracula.
I thought it was a very unimaginative treatment of vampires, and most of
the fascination of Dracula has been imbued by actors, directors, and later
"derivative" authors -- the people who dug into the myths, reinterpreted
them, and gave them more depth than Stoker did. The most enjoyment I have
with Stoker is from parodies of Stoker. The Rocky Horror Picture Show wouldn't
have been half as much fun if Bram Stoker had never invented that
thundering void Von Helsing.
But how do you define "definitive"? The book that created a category, setting the role model, inspiring other authors? The book that is the best of its category? Since everyone has their own standards for "best", the book you like the most?
The books I regard as the best horror novels are by S. P. Somtow -- but I don't believe they are known widely enough to be called "definitive".
I didn't personally get as much of a kick out of Mary Shelley's Frankenstein as I did out of The Wind in the Willows. For that matter, I found the scene in the movie Bambi where his mother was shot to be the most horrifying scene in my young life. Still, Frankenstein was at least as influential as Dracula and, I believe, incorporates more of the essence of the modern horror novel:
Utopias/Dystopias: I'm not going to reach back as far as Plato. I think that H.G. Wells was the most influential early influence, inspiring visions of both the social wonders from new technology and the possible social disasters. Edward Bellamy, while not as well remembered, was influential among those who disagreed with Wells.
Apocalyptic Fiction: After the Big Disaster: Stephen King wasn't the first one to be attracted to writing about that, by a long shot.
Thalia's Fantasy Directory