Ashley Young, Marouane Fellaini, Juan Mata and Chris Smalling got the goals as United turned in a rampant display reminiscent of their glory years under Alex Ferguson.
Yet Van Gaal still had slight cause for complaint after a sluggish start allowed Sergio Aguero to give City a deserved early lead.
The Argentina striker went on to net his 100th goal for last season's Premier League champions - scant consolation on a day that will live long in the memory for Van Gaal and his players.
"Maybe the best feeling but not the best performance because we started very badly, more or less like against Burnley, I believe," the Dutchman told his post-match press conference.
"But after the assist of David de Gea [the goalkeeper's long punt that set the move for Young's equaliser in motion], more or less we are regaining our confidence.
"We scored a goal, we scored a second goal - a fantastic attack.
"The second half was fantastic, I think."
Fellaini's muscular presence in attack repeatedly unsettled the City defence and Van Gaal enjoyed the warm ovation afforded to a player who often found himself cast as the scapegoat when United disappointed under David Moyes' management last term.
"The fans also see that he is very important for the team," Van Gaal said of the Belgium international.
"He scores goals, he gives pressure when we need it and he plays always the ball to the same colour. I'm very happy for him."
He added: "When you see how the fans are responding to the substitutions of Mata or Fellaini or Phil Jones - standing ovations; [this is] fantastic for the players, I think."
