A) "Science fiction is what you find on the shelves in the library marked science fiction"–George Hay (British futurologist)
B) "Science fiction doesn't exist"–Brian W. Aldiss
C) Narrow Wellsian definition (one change and then extrapolate)

D) Rabkin: three focal qualities:
d1: the fantastic made plausible (see "mutie shepherd" in Roger Zelazny, The Dream Master, 1966)
d2: high adventure (see E. E. "Doc" Smith, opening of The Skylark of Space, 1928)
d3: intellectual excitement (see Frederic Brown's paradigm puzzle)
After the last atomic war, Earth was dead; nothing grew, nothing lived.  The last man sat alone in a room.  There was a knock on the door . . .

Further recommended reading for those who can't stop themselves:

Isaac Asimov, Foundation Trilogy (1951, 1952, 1953; special Hugo for best SF series ever)
Frank Herbert, Dune (1965; first double award winner [Hugo and Nebula])
Michael Frayn, A Very Private Life (1968; future tense narration)
Stanislaw Lem, Solaris (1968, most critically acclaimed SF novel; see also, His Master's Voice, 1968)
Robert Silverberg, ed., The Science Fiction Hall of Fame, vol. 1 (1970; best collection of 20th century short stories)

Brian W. Aldiss, Trillion Year Spree (1986; most thorough literary history of the field)
Eric S. Rabkin, The Fantastic in Literature (1976; broad theoretical treatment of the genre)
Gary Westfahl, Science Fiction Quotations (2005;
an easy-to-use compilation from the genre)
Most widely taught SF books in universities and colleges (according to Ron Bell, New York Times, personal communication of 12 Jan 89):
Frankenstein (1818)
The Time Machine
Childhood's End (1953)
A Canticle for Leibowitz (1959)
The Left Hand of Darkness (1969)
Neuromancer (1984)

Contento's listing of the most reprinted science fiction stories
The Guardian's report of scientists' pick of best SF authors & films

Hawthorne: "The Birthmark," "The Artist of the Beautiful," "Rappaccini's Daughter," & "Dr. Heidegger's Experiment."
Poe: "A Tale of the Ragged Mountains," "The Facts in the Case of M. Valdemar," "The Pit and the Pendulum," "A Descent into the Maelström," & "The Raven"