There's always a moment of anticipation that precedes something that is anticipated. Sometimes that moment of anticipation is greater than the moment that follows, but in this case your reward will be worth all the wait. You've been waiting for 384 hours (1) which means you have used between 1,280 and 1,600 gallons of water and produced an average of 70 pounds of trash in this period. (2) But deep in the back of your minds, you all have been anticipating my reply to this next question. Finally, that reply is here.
That's right. It's the moment you've all been waiting for.
So this is a complex and intricate question, which I don't think you simplified justly. Your initial question "what is the greatest feat of fiction ever accomplished by the human mind, etc. etc." is asking a different thing than your second question "What is my favorite book." My favorite book is probably something by Brandon Sanderson, (Way Of Kings, Oathbringer, Rhythm, Yumi and the Nightmare Painter, Steelheart, Skyward, Bands of Mourning etc. etc. etc. the list goes on. (3)) However, I don't know if it would be fair to call any of his books the greatest feat of fiction ever to exist up to this point, even though I'd love to. So instead, I think it would be more fair to look to the most influential literary works ever to exist. I tried to do some research on which books were most influential, and found "The Bible" at the top of every list. (4) Which is fair, but also doesn't seem like a very good answer. So I'm going to say Lord of the Rings for fantasy, Dune for Sci-fi, Pride and Prejudice for Romance, and Sherlock Holmes for Mystery. (I'm only naming four of my favorite genres, there's obviously other genres, and this list obviously is just my opinion.
However, the book that had the greatest influence on me as a person has to be Steelheart. Maybe not Brandon Sanderson's best book, but it was what got me into his books, it was what turned me into a novelist, and those domino effects resulted in who I am today.
And now some wisdom. @Thaidakar the Ghostblood, it says in your signature that you dabble in writing, and us writers are always wondering what type of writing pays the most by word. So I have some wisdom for you, and all the rest of the writers out there. For years I wasted my time on books, and poems, and all sorts of writing that I expected would pay off, but I only recently discovered the type of writing that pays the very best. Ransom notes. I hope this wise tip saves you some time!
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You know the drill, have a good day.