Pogba? Veratti?
Also up there, as is Thiago and Koke.
Kovacic is the youngest of that lot though and I think he has the ability to be the best. I think Pogba is a bit of a highlights player, and that Verratti, for example, is better than him, as is Koke. Kovacic has strength and acceleration to go with his excellent dribbling and passing abilities, and I think he's potentially a once in a generation type of midfielder.