Wiggins is showing the most growth so far. And Jabari was playing well until he got injured. Other than that, Embiid has yet to play an NBA game, Exum is showing growth, Smart, Gordon, Vonleh, and every one else is pretty much at the same level I'd say.
It'll take a couple of years to see who the true star of the class is, but right now it's defenitely Wiggins.