I haven't read all of John Green's works yet, but so far from what I have read, this is my second favourite- The Fault in our Stars obviously being my all time favourite.
The reason I gave this book four stars was because I couldn't put in down, not in the devouring sense, but more because of a curiosity to where things were going- and I liked where they went.
The reason I didn't give this book five stars was because it simply didn't wow me. Sure! I enjoyed it and would recommend other John Green lovers to read it too, but this book didn't 'rock my world' in the sense that I was mourning after reading it.
I enjoyed it a lot more than An Abundance of Katherine's, which I found to be a little bit lagging for my taste. After I read Will Grayson, Will Grayson, I'll reassess my order of preference and let you know.