The question should be who's the smartest besides Mata. People give him shit for being slow and small etc. but he's still playing top football despite starting out with a natural handicap and at one point he was one of the best players in the PL. You only get to that level with a massive football brain.
I don't know who's behind him. Basically players that you see rarely make any kind of mistake and do the simple things well are generally the same players that have a good football brain. Honestly I think that part of the problem with our squad is that it isn't very intelligent. In our golden years we had Giggs, Scholes, Keane and Beckham. All of them were smart players. Watch City and they have the Silvas, De Bruyne, Gundogan, Fernandinho, Aguero, Laporte all of whom are smart footballers and you sprinkle them with some fast paced talent and you've got yourself a team. I think we lack that intelligence.
Matic is up there, he's an intelligent player. Made a lot of mistakes this season but who hasn't? Pogba is similar. He's an intelligent footballer but there are other parts of his brain that sometimes take control. De Gea seems very intelligent. He somewhat changed part of the goalkeeping game by showing that you can do things differently. His ability to read his opponent is what makes him such a good shot stopper, also he realized that just because he can use his hands doesn't mean you can't save with your feet. He's a player that really knows his strengths and weaknesses and that's what makes him a top goalkeeper along with his ability. Many keepers aren't top keepers because they make many mistakes but they'd otherwise be great if they didn't.
However he's still a keeper and not comparable to what an outfield player has to think about. That's why I'm giving it to Matic. Honorable mention as well to Herrera and Martial.