My preferred genre is SF, broadly described as the literature of ideas.
The best SF books make you think.
About what people do to themselves, to each other, to the world.
About "what if" or "if only!" or "if this goes on..."
It is about the world we live in and ways it might lead to a specific future.
The best SF books draw you in with interesting settings and situations, they draw you in to worlds you might want to live in or dread to experience. They suggest ways we could improve our world or warn of ways this world can go horribly wrong. Utopias, dystopias, cautionary tales.
The best SF books introduce us to interesting people; they may be just like us or, like the gods of mythology, larger than life reflections of human traits and follies. They can be people we can reasonably aspire to emulate or people we can barely understand.
The best SF books will challenge us to think. Whether dead serious or light and airy, the best SF will always tell us something worth pondering about us, our world, our role in it.
And, of course, the best SF books will do any or all of the above in an accessible, entertaining fashion. Because good SF, serious or not, has to be *fun*.