I don't think I've read enough books to give a competent answer. Besides my opinion would be subjective any way.
The Hole in the Wall by Arthur Morrison (1902) is not, I suppose, a book of great literary or scientific significance. Of all the books I have recommended to many friends over many years, this is the one that is always spoken of with greatest affection as a wonderful discovery. So I reckon that makes it the BEST.
All childrens' books are amazing. Reading them as adults is a completely different experience. I'll definitely read this baby again one of these days. My own choice: "Ender's Game" by Orson Scott Card is pretty good. Stephen King -- "The Tower" series, Tommyknockers, It, The Last Stand... I know many don't consider him to be that great of a writer, but I like him a lot.
probably not "the best book ever written".. but my favorite is Alexandre Dumas' Count of Monte Cristo