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.
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'd probably be shocked at first. After all, getting a howler is quite embarrassing.
However, if Hermione knew she was in the wrong, she might feel ashamed. But still, she would likely try to make amends in a more private and rational way. She wouldn't just take the public humiliation lying down. Instead, she'd approach Annabeth later and say something like 'I'm sorry for what I did, but the howler was too much. Let's talk about this properly.'
He might be initially shocked. Draco is used to being the one in control and having others be intimidated by him. A Howler from Percy, someone he may not have considered a huge threat before, would be unexpected. He could be angry and vow revenge in his mind.
It could be a really intense and unexpected turn. Maybe it would be full of threats and taunts from Voldemort to mess with Harry's mind.
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.
He might be initially embarrassed, especially if he receives it in a public place like the Great Hall.
Well, if Harry got a howler from Voldemort, it might be filled with dark magic and menacing words to unnerve Harry. It could also be a ploy to throw him off his guard and disrupt his focus on fighting evil.
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.
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.