Just because De Gea played every game and was protected by a good defense, doesn't mean he becomes our best player. If we had Sam Johnstone in goal last season, I bet he would have conceded max 6-7 more than De Gea did, which implies that our defense would protect even an average goal-keeper.
De Gea had nothing to do for large stretches of the games, so its' not really proper to compare a goalkeeper's starts with outfield players, as the latter are more prone to injuries and substitutions. In a few games like Stoke at home, De Gea's mistakes cost us points.
The simple question is - who was responsible for us winning the league cup? Zlatan. Who was responsible for keeping us in a top 4 race atleast till April? Zlatan. Who won us the EL? Mkhi's goals throughout the campaign.
Did De Gea make super saves to keep us in games and win us vital points like the previous seasons? No.
Zlatan, ergo, was our best player. His barren run is exaggerated, it was for a month after which he was scoring consistently. Its' the others who missed chances.