Like with any story, a key part of what makes a particular anime great is the character growth seen in the series. Whether it is after a fall from grace or a character that has to survive with all the odds stacked against them, the way a character changes as a story progresses is what keeps anyone coming back for more.
And one of the typical figures that makes sure this growth takes place is a mentor or teacher. Sometimes they're harsh and rude but give characters the kick they need to continue. Other times, they're emotional and just as unsure as the character they're mentoring, but they make sure they're taken care of. Whatever side they fall on, they do have the best interests of the main character at heart (most of the time)!
Jujutsu Kaisen's strongest character, Gojo Satoru, is a teacher at the Tokyo school for sorcerers. With an interesting character design, incredible powers, and quirky personality, Gojo seems like he'd be a fun teacher to have. But his students tend to disagree.
While he does care about them and is taking the time to teach them, Gojo does also have ulterior motives for training his students. Plus, in his day-to-day, Gojo is shown to be narcissistic, trick his students, take pictures of them when they're down, and act so silly that he is quite often vocally disliked by all due to his inability to read a room (or maybe he just doesn't care).
The thirteenth student of the Hiten Mitsurugi style, Hiko is the strongest swordsman in a series many consider to be the best samurai anime of all time, Rurouni Kenshin. He is never seen to be beaten in the entirety of the series. Like Gojo, the author considers him to be such a powerful character, that it is difficult to write for him and include him in the story.
He is shown to be sarcastic, vindictive, and egotistical when training Kenshin, and has to be convinced to be available to protect Kenshin's friends when his apprentice asks. Despite all this, it is understood that he is trying to make sure Kenshin lives the life he wants, and that, in his own way, he does care about some people in his life.
While Reborn starts out as a hilarious, quirky comedy series with fun powers, the shift in tone to a standard battle shonen series also shifts Reborn's responsibilities. The original premise focused on the hilarity of a baby mafioso shooting at a future mafia boss and wearing a variety of costumes paints Reborn as quite the harsh, uncaring, and sadistic teacher for Sawada Tsunayoshi.
By the time Tsuna is willingly training to protect those he cares about, it becomes clear that Reborn does care about his young student. While it is very clear he does also enjoy making him suffer, it is shown time and time again that this student-teacher duo cares deeply about each other, and that Reborn's presence in his life, while initially unwanted, helped Tsuna immensely.
This is a case of the main character being the teacher. Denpa Kyoushi's Junichiro did not set out to be a teacher himself, and indeed at the start, is not exactly happy at suddenly becoming one. While he is indeed very smart, he is not the best at interacting with others.
As the series progresses, Junichiro begins to enjoy teaching. He is able to break down physics concepts to be understandable to his students, weaving his own interests in gaming and anime as part of his lessons, and inspiring his own students to pursue their own studies.
Onizuka tells a story of redemption. As in Denpa Kyoushi, it is the teacher that is the main character in the '90s anime, Great Teacher Onizuka, as he learns to navigate the highs and lows of his new profession, not always in the most graceful or appropriate of ways.
Onizuka is likable precisely because of all the chaos he causes in his wake. Because the chaos has a reason, and that reason is his students. It is here that Onizuka shines since he himself comes from a similar background and wants better for these students, even if he has to drag them kicking and screaming to it.
Our first introduction to Demon Slayer's Urokodaki is when he is disappointed by Tanjiro's lack of resolve to kill a demon. He then begins taking care of the siblings, while putting Tanjiro through grueling training, preparing him for the Final Selection to become a Demon Slayer.
While Urokodaki is certainly a fierce fighter and trainer, there is a constant undercurrent of care for his students. And that care is clearly reciprocated, as despite being killed his students have been sticking around after death. For a character with a rather small role in the awe-inspiring world of Demon Slayer, which went on to be one of the best movies of 2020, Urokodaki's likeability as a teacher shines through in his students.
Aizawa is a poster child for gruff, lonely characters that say they don't care but actually care a whole lot. My Hero Academia reminds its characters that they are, in fact, children. And no one reminds them more of this than Aizawa, who, in his role as teacher, is constantly looking out for them and putting himself in harm's way to make sure that his students are safe.
Aizawa's seemingly uncaring attitude hides how much he actually cares about his students, his fellow heroes, and his job. As an underground hero, most of his work is thankless, and he views his work as a teacher similarly. Despite this, everyone around him clearly likes him a lot.
Naruto's Kakashi deserves his spot as a likable teacher precisely because he did not set out to be one. With one of the most tragic backstories in the entire series, Kakashi is first introduced as a mysterious, powerful figure, at best apathetic about having to train a trio of 12-year-olds.
While a lot of Kakashi's teaching has come into question, his likability can be seen as so many characters rely on him and his expertise throughout the series. From Naruto to Gai to Obito, he impacts them all. There is even an entire movie dedicated to how little Kakashi cares about himself, and how much everyone else cares about him.
Haikyuu!! is sports manga at its best: a small, underdog team made up of a bunch of misfits takes on teams with more experience, somehow scraping by each time. It is impossible not to cheer for the Karasuno team as they do better and better, and Takeda Ittetsu is definitely cheering.
The academic advisor for the team might not know much about volleyball, but he is determined to help them succeed. From getting them the right coach, good practice matches, and providing the necessary speeches at the right time to fire up the team, there is no doubt that he is part of the secret sauce that keeps the team running and successful - and likable too.
Unsurprising given Korosensei's unique grip on the teacher stereotype, Assassination Classroom's teacher is easily the most likable teacher in anime. Korosensei fulfills his promise of helping Class E, a class of purported losers, achieve and surpass their potential, to the surprise of the rest of the school and the students themselves.
Placed against a backdrop of assassination, the destruction of the moon, and the end of the world, Korosensei nonetheless achieves what had been previously dubbed impossible. His prowess and dedication as a teacher, best exemplified by his grading tests while going at his terrifying speed traveling between countries, is comparable to none.