I find it funny how people dismiss Smith. The bloke has already scored tons in England, SA and the Windies. He's averaging mid-fifties after 30+ tests. He'll probably end with better stats than Ponting.
That said, it's still early for him, Kohli, Root, and Williamson. I actually agree with the post above, and think the latter of the four is probably the best in terms of technique. I think Williamson will have the highest ceiling. This debate is largely superfluous, though. They all have the potential to be the best of their generation (maybe not Kohli, actually, I find him difficult to analyse) so let's just see how they develop over the next decade.
What I do know - after the culture of Cook etc. - is that we're exceptionally lucky to have that quartet, and the likes of ABDV. What an exciting time to watch all three forms of the game!