I would like to mention that The Fault in Our Stars is Green's most recent novel, so obviously that is the best one.
Green's an average author, but he relies too much on themes from other people. Looking Alaska had Rabeleis' "great perhaps" and Bolivar's "labyrinth" as it's primary themes. Paper Towns had Whitman. The Fault in Our Stars is an obvious reference to Shakespeare. When this man is talented enough to come up with his own amazing themes (or, rather, being able to explain them without relying on the words of others), then he might be a little better than just average. That being said, I'm glad he made a little progress with his latest novel by ditching the bland characters he loves to use and reuse.