Definitely Hermione. Her intelligence and vast knowledge of magic would be a great asset if cloned. She could help solve complex magical problems and come up with strategies in battles.
In the 'Harry Potter makes a clone army fanfiction', perhaps Harry creates the clone army to protect Hogwarts. He might use some ancient magic spells he discovers in the library. These clones could be modeled after the greatest wizards in history, like Merlin. They would bring a whole new level of defense to the school against the dark forces.
He could have found a magical artifact. There are many artifacts in the Harry Potter world that have strange and powerful abilities. It could be that this artifact has the power to create clones. He figures out how to use it, perhaps with the help of Hermione who is good at researching such things. And then he starts creating his clone army.
It could also change his relationships. His friends might be worried about this new and rather extreme power he has. They might question his motives and whether this is the right way to deal with the problems in the wizarding world. On the other hand, his enemies would definitely be more cautious around him.
The originality of the story. Harry Potter has a unique plot that follows Harry's growth and his battle against Voldemort. This is not a recycled or cloned idea.
Obviously, his friends Ron and Hermione would be among the first to join. Neville could also be a key member, as he has shown great courage. Then there might be the members of Dumbledore's Army who are still at Hogwarts. The Weasley twins could also be part of it, using their unique magical inventions to give the army an edge in battle.
Neville would likely be one of the first to join. He has always been brave and has a strong sense of justice. Luna Lovegood might also be part of it. Her unique perspective and knowledge of the wizarding world could be very useful. And then there are the Weasley twins. Their skills in magic and their ability to think outside the box would make them great assets.
Obviously Naruto himself would be a main character as the source of the clones. Maybe Sasuke could also be involved, either as an ally or an antagonist in relation to the clone army.
In this fanfiction, Harry might also include some of the Hogwarts ghosts in his army. After all, they have a long - standing relationship with the school. Peeves, for example, could be a wild card in the army. He can cause chaos among the enemies. And some of the older students who have a strong sense of justice, like Cedric Diggory's friends, could also be part of it.
Hermione Granger might also be a key character. She could be involved in researching the theory behind the shadow clones and helping Harry control or improve this new form of magic.