Korosensei is the most interesting without a doubt. His appearance alone is striking, and his ability to teach while being targeted for assassination is quite remarkable. Karma Akabane is also very interesting. He's a bit of a troublemaker with a high intelligence and his interactions with other characters add a lot of spice to the story. Then there's Kayano Kaede. Her true identity and her role in the story are full of mystery and make her an interesting character.
Koro - sensei is definitely one of the most interesting characters. He's an octopus - like alien who is supposed to destroy the Earth but instead becomes a teacher. His superhuman abilities and his unique teaching methods make him stand out. Nagisa Shiota is also interesting. He has an unassuming appearance but is very perceptive and has some great assassination skills.
Nagisa Shiota is also very popular. Fanfics could focus on his growth as an assassin and how his initially meek personality evolves over time. His unique skills and his friendship with other classmates can be great elements to build a story around.
Nagisa Shiota from 'Assassination Classroom' can also be quite popular in these crossovers. He has his own growth arc in the original series. In a fanfic with Harry, he could learn from Harry's experiences at Hogwarts and vice versa. Their friendship could develop in a way that combines the best of both worlds, with Nagisa's assassination skills and Harry's magical knowledge.
Definitely Koro - sensei. He's a very unique character as he's an octopus - like teacher who the students are supposed to assassinate. His abilities and his rather jovial yet mysterious personality make him stand out. Another interesting character is Nagisa Shiota. He has a gentle appearance but is quite skilled in assassination, which is an interesting contrast.
Nagisa would be a great character to pair with Okuda. Nagisa is very observant and could help Okuda in her experiments by noticing small details that she might miss. Also, their different personalities would create an interesting dynamic. Karma could also be a good pairing. He's very intelligent in his own way and his mischievous nature could either help or hinder Okuda's scientific pursuits, leading to some fun and tense situations.