In terms of all around midfielders there aren't anyone else even close to him the way he's been playing for the past couple of months. He does everything form shutting the opposition down defensively to starting our attacks and finishing them off.
There are a couple of players who have provided more end product thus far this season, so I suppose you could argue that they contend with him for "best midfielder in the league" (even if none of them are traditional midfielders in the sense that he is). I suspect it won't be long until he's matching their numbers in terms of direct contributions as well, and at that point there won't be anything to debate.
One thing I know for certain is that there is not a player in the league I would swap Pogba for, midfielder or otherwise. The boy is 23 and his ceiling is sky high, and the prospect of him staying with us throughout his best years fills me with glee. I have a feeling those £89 million will end up looking like something of a bargain.