"Erm, NOOO!!!! what the sigma that's incorrect!111!!!!"

I like arc number two the most

Well your not wrong about that but logically the second arc would've never been possible without the first. So the first is the best.

Most parts of the first arc wouldn't have been possible without the Legends books.

Because, like, NightWings would have never needed to make up the prophecy of the 5 dragonets without being desperate for a home. And Darkstalker existing and going evil made the NightWings find a new miserable home to live in.

So the Legends: Darkstalker is the best book.