Hmm, it's hard to say what constitutes the best book I've ever read since I've read so many stories that have been considered as "The best book" at the time of reading it. However what was my favorite book at my teenage years is not my favorite book today, but does that mean I should relegate the old favorite to a lower position? Rather I think the best solution is to mention my favorite books throughout the ages. So I'll list them by my age range when it was my favorite.
Favorite book/series as a child - Animorphs
Favorite book/series as a teen - Harry Potter
Favorite book/series in the military - Sword of Truth Series
Favorite book/series in college - Drizzt the Drow Elf/Forgotten Realms series
Favorite book/series post college - The Magicians series by Lev Grossman
Favorite book/series currently - Coiling Dragon (Chinese fantasy novel).
My favorites have changed as I grew up. As a child the first book I checked out of the school library was Where The Sidewalk Ends. I devoured it over the weekend and asked for it as my birthday gift.
Then there was the Wizard and Warrior choose your own adventure books. Then Where The Red Fern Grows, The Jack London books and anything Cryptizoological in nature.
When I was 10 my mom put a King book in my hands and I was hooked. It opened up the horror genre and I found Barker, Lovecraft, Poe and others. I read my first Forgotten Realms book at 17 and fell in love with the Drow and Menzoberronzan.
Then came Milton, Dante', Nietzsche, Voltaire, Socrates and Twain.
I love literature. Can't pick a favorite.
Just about anything by Terry Pratchett. But I think "Guards, Guards", "Lords and Ladies", "Fifth Elephant", "Interesting Times", and "Small Gods" give a great overview of the mix of humor, insight into human society, and love and sympathy for people that Pratchett displayed. They are the most self-aware books, without being pretentious, that I have ever read. And they are often truly laugh out loud hilarious.
It's always very difficult to chose one book to carry that title! But if I had to chose only one, considering one that really impact me, is a Brazilian one called "Meu Pé de Laranja-Lima".
I believe its name in English was translated to "My Sweet Orange Tree".
It tells the story of a very imaginative boy and how he handles the fact that his family needs him to start wondering around and face the "Real world". It's very sad, but beautiful.