Best formation for Man Utd is lined up against a wall and me six yards away with a semi automatic
Seriously though a 442 should work okay. Because you have such good wingers in Giggs and Ronaldo who like to run at players rather than hug the touchline you could try playing a narrow short passing game pushed up the park rather than having your wingers wide because those two are less effective as true wide men. If you have one of your MC's as attacking/creative then you should form a strong attacking bias while keeping the defence reasonably tight.