Philip K. Dick

Philip K. Dick's novels have in them the primary characteristic of great literature - psychological insight and an understanding of the human condition.  In fact Dick was in his younger days fascinated with psychology, and a knowledge of it permeates his work.

They also have that characteristic of science-fiction that you don't get elsewhere - an attempt to illustrate the nature of reality by examining "what if" scenarios.  With a wild degree of imagination.  And in Dick's case, a conscious attempt to at least sometimes reflect the anarchic nature of the universe rather than to stick to tried-and-true dramatic structures.

No one else, that I've ever read, managed to do both of these things well at the same time.  And to interleave them into a philosophical picture of what reality is, while delineating strong human characters.

If you consider yourself a "serious" reader, and want to read his most serious books, I recommend "Martian Time-Slip" and "A Scanner Darkly".

Beyond those, if you want to read great books full of imagination and depth, I recommend (in order of preference) :

The Penultimate Truth
We Can Build You
The Three Stigmata of Palmer Eldridge
The Crack in Space
The World Jones Made
Ubik (either the novel or the screenplay)
A Maze of Death
The Man Who Japed
Now Wait For Last Year
VALIS
The Divine Invasion
The Simulacra
Vulcan's Hammer
Dr. Futurity
Galactic Pot-Healer
Counter-Clock World
The Man in the High Castle
Dr. Bloodmoney