Great question. Always a tough one - look at those silly "combined 11" lists people do, some of them had 11 tottenham players in them in the lead-up to our game, yet we beat them, so it's folly at times. But if I'm thinking seriously about who I'd have in our squad, then:
De Gea
Sanchez
Shaw (long-term replacement for Monreal)
Pogba
Martial
Rashford
If ManU were playing as well as they could, more players would be in form and would make this list. For example, when Bailly started I thought he looked fantastic, but he's regressed terribly. I would take that early version of Bailly at the drop of a hat. I'd have taken an in-form Matic had we not signed Torreira too.
On the other side of the coin, who from Arsenal would you have in your ManU squad?