My app was featured on the App Store as "Game of the Day" and "Great on iPhone X" and has gotten pretty popular: https://itunes.apple.com/app/id1312458558

It's called Rainbrow and it uses the iPhone X's face-tracking capabilities to allow you to play the game with your eyebrows. Not exactly an ARKit app in the traditional sense, but it makes use of ARKit API's. It's also free!