These are my favorite SF books. They are listed in alphabetical order. As you can see, I'm stuck in the past:
Adventures in Time and Space (anthology)
Raymond J. Healy & J. Francis McComas, ed.
Animal Farm
George Orwell
Cat's Cradle
Kurt Vonnegut, Jr.
Childhood's End
Arthur C. Clarke
City (collection of related short stories)
Clifford D. Simak
The Demolished Man
Alfred Bester
The Door into Summer
Robert A. Heinlein
The Dreaming Jewels (AKA: The Synthetic Man)
Theodore Sturgeon
Ender's Game
Orson Scott Card
Fahrenheit 451
Ray Bradbury
The Haunted Stars
Edmond Hamilton
Lucifer's Hammer
Larry Niven & Jerry Pournelle
The Mote in God's Eye
Larry Niven & Jerry Pournelle
Nineteen Eighty-Four
George Orwell
The Pocket Book of Science Fiction (anthology)
Donald A. Wollheim, ed.
The Power
Frank M. Robinson
Shadows in the Sun
Chad Oliver
Slan
A. E. van Vogt
The Stars, My Destination
Alfred Bester
The Third Level (short story collection)
Jack Finney
To Your Scattered Bodies Go
Philip José Farmer
The Twenty-Seventh Day
John Mantley
Way Station
Clifford D. Simak
The War of the Worlds
H. G. Welles
The Winds of Time
Chad Oliver
My goodness this is difficult. Tastes vary and people's personal circumstances often alter the relative significance dramatically. One book that is experienced as life-changing for one person leaves another phlegmatic and meh.
That being said, it's difficult to imagine any list of great science fiction / fantasy books which does not include at least these:
The Silmarillion
JRR Tolkien
(1)The Fountains of Paradise (2) Rendezvous With Rama
Arthur C. Clark
The Weapons Shop (of Isher)
AE Von Vogt
(1) Green Hills of Earth (anthology), (2) Stranger in a Strange Land
Robert Heinlein
Tuf Voyaging
George RR Martin
(1) The Left Hand of Darkness, (2) A Wizard of Earthsea(trilogy), (3) Four Ways to Forgiveness, (4)The Word for World is "Forest"
Ursula K. LeGuin,
The Space Trilogy (Out of the Silent Planet, Perelandra, That Hideous Strength)
C.S. Lewis
The Severian Series (Shadow of the Torturer, Sword of the Lictor, The Claw of the Conciliator, The Citadel of the Autarch, Urth of the New Sun) plus The Other Dead Man (short story), The Island of Dr. Death (short story).
Gene Wolfe
(1) A Specter is Haunting Texas, (2) Ill Met at Lankhmar
Fritz Leiber
(1) The Sirens of Titan, (2) God Bless You Mr. Rosewater (3) Timescape
Kurt Vonnegut
A Canticle for Leibowitz
Henry Miller
A Voyage to Arcturus
David Lindsay
(1) V.A.L.I.S., (2) Ubik, (3) Do Androids Dream of Electric Sheep? (3) The Three Stigmata of Palmer Eldritch
Philip K. Dick (aka Horselover Fat)
The Martian Chronicles
Ray Bradbury
The Stainless Steel Rat
Jack Williamson
(1) Neuromancer (2) Virtual Light (3) Idoru (4) Pattern Recognition
William Gibson
The Gods Themselves
Isaac Azimov
(1) Dune (2a) Destination:Void (2b)The Jesus Incident (2c)The Lazarus Effect (2d)The Ascension Factor (3) The God Makers
Frank Herbert