What is the most popular book by Rick Riordan?

Percy Jackson and the Olympians

book 1: the lighting thief

book 2: the sea of monsters

book 3: the titans curse

book 4: the battle ofd the labyrinth

book 5: the last olympian

the Kane chronicles

book 1: the red pyramid

the heroes of Olympus

book 1: the lost hero

And the maze of bones (the first 39 clues book)

also the Tres Navarre mystery series for adults