I don't own muscovies, but I know a little about them, and I own mallard ducks, so here's my opinion:
While I agree that the pack/flock mentality is the problem, I also believe amykins' way of thinking will make it worse.
Look, your muscovy sees himself as a duck. He also sees you as a duck. He knows you are the biggest, and also the leader. He's essentially jealous. It's not the kind of complex jealous feeling that humans get, but it is still a very simple, primal form of jealousy. What he's doing is challenging you for the spot in the pecking order, a spot he feels entitled to.
You can do two things.
1. "fight" with him and win, like amykins suggests. The problem I see with this is that he will eventually pluck up the courage to try again, and again, and again. You can see this yourself; you said it worked for your other drake, but not this one. He's very determined. Ducks aren't the most logical of creatures and their memories are not great. He's operating on hormones, not war strategy LOL. You can beat him, but sooner than later he will forget about it and try again. (If you do do this, I wouldn't yell at him. It's just cruel IMHO. He won't understand what's happening, only that you're attacking and scaring him. Muscovy males don't yell at each other when they fight. Yelling will remind him of a predator growling, not another duck. Then he'll see you as a threat to his flock and will either A- remain aggressive in a protective way, or B. become skittish and avoid you.)
2. Convince him you are NOT a duck. This might be impossible if he imprinted on humans but you can try. Try "backing off" from the flock. If any of the girls "Squat" for you, ignore them. Don't pet them; that just reinforces your drake's belief that this is YOUR flock and if he can beat you it will belong to him. Make him believe he already IS the boss and doesn't need to fight. Keep talking to your flock, but tone it down- don't respond if a duck "talks" to you. You want him to see you as a "pet" of sorts LOL. "You're not a duck. You're just some strange creature that brings us treats."
If you can do 2, he might calm down. Otherwise, you might have to eat him. It wouldn't be right to give him away if he attacks people. Is it just you he's attacking or everyone?
I don't think you babied him too much, since it's just this drake in particular, as you said your others are fine. It's just a mixture of genetics and hormones. Muscovy drakes are like roosters in that way. Sometimes they are just aggressive by design.
Hope this helps.