Harry could be angry at first. He might think his parents are overreacting or don't understand his situation at Hogwarts. But then, after thinking about it, he realizes they are just worried about him and starts to feel guilty.
He might be initially embarrassed, especially if he receives it in a public place like the Great Hall.
In some fanfictions, Sirius might use it as a chance to show off his independence. He could read the Howler out loud in front of his friends, laughing at his mother's old - fashioned views. This would show that he doesn't care about her threats and is proud to be different from the rest of the Black family.
Voldemort might initially be furious. He's not used to being challenged in such a blatant way. He could try to destroy the Howler immediately in a fit of rage.
He might be very embarrassed. Howlers are known for delivering their messages in a rather loud and public way. Draco, who is used to maintaining an image of superiority, would find it humiliating to have his family's business aired out in front of his classmates.
Harry might be initially shocked and embarrassed. A Howler is a very public form of scolding, and in front of all his friends and classmates, it would be a humiliating experience for him.
Well, it could be about him getting into trouble at Hogwarts. Maybe they scold him for sneaking out at night or getting involved in a dangerous adventure without permission.
Well, it could lead to all sorts of drama and revelations. Maybe it's a scolding for something he did or a message of love and concern.
Harry might immediately gather his friends and share the contents of the Howler with them. He values their input and they've always been there to support him. Together, they would analyze the message and come up with a plan. Maybe he'd be a bit angry too, feeling that Voldemort is always trying to disrupt his life and the safety of those around him.
He might be initially shocked. After all, getting a Howler is a very public and embarrassing thing. He could be so ashamed that he immediately hides away to read it privately, his face turning red with a mix of anger and humiliation.
Harry would probably be overjoyed at first. After all, he has always longed for his parents. He might start to look for ways to make it happen, like searching for magic spells or ancient artifacts that could bring his parents back.