"Stranger in a Strange Land" by Robert Heinlein.
This review from Amazon sums it up well:
"Is it the best science fiction novel ever published? I would say yes. Is it my favorite? No; it's not even my favorite Heinlein novel. To add a little more irony to the pot, Heinlein himself insisted that the book is not really science fiction to begin with. Stranger really marks a huge turning point in Heinlein's career. Unhappy with the brand of "juvenile" writer and the editing that position constantly entailed, Heinlein was determined to write a truly adult novel, one with no taboos, no limits, and no restrictions of any kind. With Stranger in a Strange Land he accomplished that in spades, basically taking on the heretofore sacrosanct subjects of sexuality and religion. Heinlein was not sure that anyone would even publish this story that took him 12 years to write; what was published was a mere figment of the original manuscript, 60,000 words having been cut out. Even though Heinlein did the editing himself, it had to have felt like jabbing an ice pick into his own heart to do it. Thankfully, we can now read the complete, original manuscript the way Heinlein intended the story to be told.
The plot is deceptively simple. The first manned mission to Mars never made it home to Earth. The second mission, twenty years later, found Valentine Michael Smith, an infant born on Mars and the only surviving member of the ill-fated first mission. Having been raised by Martians, Smith is literally a stranger in a strange land when he is brought back to earth with "miraculous" abilities and a Martian philosophy of life. The Federation government basically hides him away from prying eyes, partly in fear of the legal and political dangers posed by his unique status. Having been raised by Martians, the human experience is completely new and rather frightening to him. He has never even met a woman until nurse Jill Boardman sneaks into his room to get a glimpse of him. Fearful that the government is going to keep Michael basically imprisoned (or worse), Jill helps sneak him out of the hospital, and the two of them end up at the home of Jubal Harshaw. Jubal is an outspoken, older man who lives a thoroughly individual lifestyle, but he commits himself to helping Michael escape his perilous situation. Michael quickly begins to absorb human knowledge and, less quickly, begins to understand the confusing mentality of human beings.
Halfway through the novel, you may be asking yourself why the book was so controversial; the answer becomes clear as Michael now steps out into the wider world. He and Jill move around incognito, and Michael learns more about people. After a stint as an unsuccessful magician, he eventually decides to become a preacher. He's not preaching a religion, though; he offers humans a new way of living and thinking, one based on the Martian system he grew up in. This new lifestyle involves a lot of nudity, a lot of open fornication, and the constant repetition of a mantra of sorts naming yourself and those around you God. The "I am God, you are God" theme is essentially Heinlein's means of emphasizing the personal responsibility of each individual for his own life. It is not strictly antireligious, but certainly it is not an idea that would go over well among most fundamentalists. I say most because I am a fundamentalist myself, but I understand what Heinlein was saying and recognize the fact that, after all, this is fiction. Frankly, though, the free love theme bothers my sensibilities and causes my viewpoint of the novel to change somewhat. Even though disapproval began to temper my enthusiasm toward the end, I certainly cannot give this book less than five stars.
Science fiction readers had never read anything quite like Stranger back in 1961; its originality, bold themes, and fearless writing hit with the force of a hurricane, and science fiction has never been quite the same. The Hugo Award this novel rightfully won barely begins to give it the honor and acclaim it deserves. I cannot recommend this novel highly enough, albeit I must enclose a caveat with my endorsement. This book has the power to shock readers even today; do not let your own beliefs take away from the wonder to be found in the pages of this novel. Stranger requires and deserves a completely open mind from anyone who would approach it; it also requires multiple readings to even begin to plumb the depths of its riches."
I probably would have voted for "1984" by George Orwell (Eric Arthur Blair), but I'm not so sure it can be classified as "fiction" these days.