The question of what one's favorite book is has always struck me as a difficult and possibly for many people, an unanswerable question. I love genre fiction, so I could give the very flip answer of "all of them", but that wouldn't be true. Although there are not many, there are a few genre novels that I have read and simply did not like. A couple were even offensively bad. One or two were simply offensive. The more salient point is that saying "all of them" is not merely false, it is simply not interesting. Anyone who reads this blog for any amount of time knows that I love genre fiction, so saying that all (or even most) such novels are my "favorite" doesn't actually tell anyone anything of note.
I have done this before, so I will do it again: I'm going to cheat a little bit and pick two novels as my favorite: One science fiction novel, and one fantasy novel. This is because although the two genres have some overlap in places, and traditionally many authors such as Andre Norton and Poul Anderson happily flitted back and forth between them, the two genres are by and large so different that it is simply impossible to compare the best of the one with the best of the other.
A Wizard of Earthsea by Ursula K. Le Guin
But while books like The Lord of the Rings, The Book of Three, and Elric of Melnibone transformed me into a fantasy fiction fan, it was A Wizard of Earthsea that showed me that fantasy could involve something other than pseudo-Arthurian knights running from castle to castle while they fought the onrushing tide of evil humanoids. Or undead. Or (in the case of the Elric of Melnibone series) were a direct counter reaction against those sorts of fantasy depictions. The world presented in the Earthsea books was entirely different in tone - the most obvious being that non-white characters dominated the stories, while the white characters were all dangerous and bloodthirsty barbarians from the north. But the differences were more profound than mere changes to the skin coloration of the protagonists. The stories were focused on study and learning rather than swords and heroism. In A Wizard of Earthsea, the primary antagonist is created in a moment of thoughtless hubris, and to defeat his foe the book's main character has to think his way through the problem rather than acquiring martial or even arcane prowess. Understanding, not force of arms, is the key to victory in Earthsea. In the book, Sparrowhawk defeats the dragon Yevaud, by talking to it. In short, Le Guin created a fantasy that was more interesting and more thoughtful than any other that I have ever read, and that is why it is my favorite of the genre.
Nova by Samuel R. Delany