Starting this (even if I have another couple of hours until October 1st. This will also be crossposted on my blog.)
This isn't too difiicult for me to answer since I had a book superlatives post of last year. I had several favorites from different categories, but I'm assuming that this is asking me what one book stood out to me among all the rest.
I'll say "Ready Player One" by Ernest Cline because it seriously blew me away. I loved the audio narration by Wil Wheaton, and I remember being completely invested in the book for the entire time I read it. It's interesting that I was just thinking about this book when responding in a comment on my blog about pop culture references. The ones that were done with respect to this book really stood out to me, as they were woven into the story and served a purpose. I loved it.
Second would be a non-fiction book: Susan Cain's "Quiet" - I can't stress how much this book described my experience as an introvert, and I loved the breakdown of the different dimensions of personality type in our society, as well as how people's experiences of it were concerned.
If you twist my arm enough respectively, I'll say "Wanderlove" by Kirsten Hubbard comes in third overall - because it was the kind of coming of age story that I loved and I haven't come across a YA book that incorporated travel, self-discovery, and romance rolled into one. Loved the characters and it made me want to be a "global vagabond."