Episode 1 of the anime: Yes, it easily crosses over into Nightmare Fuel, and it firmly cements Koro-sensei as an anti-villain, but his outrage and subsequent threat to the three delinquents who had the gall to put Nagisa in danger just to take a swing at him is nothing short of amazing. Beware the Nice Ones indeed. The Nightmare Fuel may have been mitigated by the fact that he was bluffing.
Koro-sensei: Here's the deal, kiddos...If you pull another irresponsible stunt like that again...there's nothing that says I can't harm someone else. Family...friends...everyone in the whole world if I feel like it!
In chapter 11, two of the normal students try to bully Nagisa. Nagisa simply laughs them off and sends them running back to their teachers.
Koro-Sensei going so fast to help each student individually so they can score at the top 50 for their next exams is pretty cool to see (and heartwarming).
Many of Koro-Sensei's actions are either this, CMOF, or CMOH. Frequently, multiple crowns apply to the same actions.
Class E's transfer student Ritsu, a learning AI, packed to the circuit boards with guns, blowing off Korosensei's index finger by repeating her previous attack exactly, and hiding a bullet behind the one she knows he'll deflect.
Her introduction doesn't sit well with the rest of the class, as she completely disrupts the day's lessons by loudly firing her weapons on Koro-Sensei all day. It's Terasaka of all people who puts a stop to it, by duct taping the ports for her fire arms shut.
New transfer student Itona Horibe, lopping Koro-sensei's arm off.
Chapter 36, Class E manages to beat the Baseball Club. Despite the Chairman's helping them.
Chapter 38. Nagisa's killing intent is so strong and he moves so quietly and quickly, Karasuma has a vision of a viper, coiled around his neck and already poised to strike. Karasuma, The Stoic extraordinaire, panics and knocks Nagisa away with his full strength.
Then in Chapter 40, Karasuma chooses Nagisa to fight Takaoka, the new Sadist Teacher on the block, as a direct result of Chapter 38.
Then in chapter 41, the foreshadowing bears fruit. Nagisa is explicitly mentioned to have a great talent for assassination, because of his talent to control his bloodlust, to frighten others with bloodlust alone, and to be at ease when acting in front of an audience. He demonstrates this by taking down Takaoka with scornful ease, while looking like he was just walking to school. He even shocks Karasuma by far exceeding his expectations, which he amusingly misinterprets as Karasuma expecting him to use the sharp side of the blade, suggesting that he truly had no compunctions against using a real knife against a person.
Capped off in chapter 42, when Takaoka is raging at Nagisa for a rematch, promising to beat him to a pulp. Nagisa simply bows, thanks him for his hard work, and politely asks him to leave. Kid's got a spine of steel when he's in the mood.
And Karasuma pwned Takaoka◊ with one hit while not even looking at him.
For the final touch, the Chairman shows up and firing Takaoka.
Class E defeating Itona. By way of a second day worn shirt covered in Anti-Sensei powder, and a giant cannonball into the pool.
Special mention goes to Terasaka whom is durable enough to withstand Itona's tentacles slamming into him and staying conscious, he's even able to stop them.
The whole chapter 53. Tests have never being so epic! To summarize, we have the tests as giant armors which hide a lot of traps beneath and the two classes battled them with spells, hammers, Naginatas, and guns.
Before that, there was Chapter 14, where answering the questions on the midterm tests are regarded as something akin to slaying gigantic, carnivorous monsters.
Yuuma, Manami and Rio taking the number 1 spot in Social Studies, Science and English from Asano.
During the first year finals, Class E manages to completely trounce Class A's test scores and monopolize most of the top 50 spots in the school. They also pull one over on Koro-sensei by having a bunch of their weaker students all get the top scores in home economics. As part of the agreement they made with Koro-sensei, their Loophole Abuse allows them to get a huge handicap next time they fight him. They also manage to take Class A's special summer resort trip.
Koro-sensei explains later that this is even more impressive than it appears. Home Ec can be a tricky subject because the tests are very subjective (represented by showing how Koro-sensei cooks with sesame seed oil while the main campus teacher prefers olive oil) and therefore the four students who got top scores in it probably worked their tails off studying past exam trends to make sure they could cook in a way that would score well for a teacher who's not Koro-sensei.
Regardless of its actual effectiveness, the sheer complexity and scope of Class 3E's plan in chapter 59 is pretty awesome and shows how much they know about their target, and how serious they are. The only reason they failed was because they were 0.5 seconds too slow in the delivery. If they'd been just slightly faster, even Koro-Sensei's ultimate defense would have been useless.
Koro-sensei sets up a plan that even the pros think is suicidal and impossible. Largely because it involves the students scaling a cliff. Just as Karasuma is ready to give up he looks up to find... the class is already halfway there. Class 3-E are now ninjas.
It's later revealed that they possess Ninja sneak walk techniques.
Karma in chapter 64 and 65. This kid is unstoppable.
Koro-Sensei and Class-E defeating Gastro in chapter 68. With a badass tactic of confusion and bringing the stage lights down on him.
Little Nagisa making a clown out of Takaoka again in chapter 72, with a technique derived from sumo wrestling no less. Then he (inadvertently?) inflicts mundane Mind Rape on the guy by smiling as he knocks him out.
Notable panel: execution-style tasing.
Terasaka should get some credit, too. Not only does he ignores the effects of a deadly virus for hours while others were knocked out after ten minutes, his words of encouragement allow Nagisa to snap out of his rage-induced breakdown.
Chapter 79. All of Chapter 79. Takebayashi just became a Memetic Badass.
To expand upon that, Chairman Asano gave Takebayashi the chance to get a HUGE boost in power and respect by giving a speech lying about Class E, deriding them, and saying that he wanted to create a separate class that would "reform" them. He seems to consider it but, instead, the following happened when he was on stage to give his speech to the whole school:
Takebayashi: In Class E, in which I stayed...is a gathering of weak people. Because they lack what we call Academic Strength, they receive discriminatory treatment from everyone in the Main Building. But, for me, Class E is almost as comfortable to be in as a Maid Cafe...I was lying. Because I wanted to be strong. Because I wanted to be accepted. But, even though I was the most useless among Class E, and betrayed them on top of that, my classmates still came to see how I was doing, many times. Sensei, even for a student like me who had no knack for studying - he tried every method possible and schemed to teach me. I, who was unaccepted by my family and all of you, was treated as an equal by Class E. All of you, who are aiming to become distinctly strong and accepted by society...I believe you are right, and I respect you. But, for a little while longer, I'm fine with being weak. While bearing with weakness, and enjoying weakness, I will return to my life of aiming for the necks of the strong. [he is scolded by Gakushuu, and pulls out the plaque from the chairman's office] I swiped this from the Chairman's room. It is like a shield which commends a private school's best manager...The Chairman is truly a strong person. All his actions are rational. [he smashes the plaque to pieces, using a knife] According to Asano-kun, in the past, another student did this same thing, or something like that. Thinking rationally, based on precedent, I'd say it's off to Class E for me as well. [he starts to leave the stage] Gakushuu: Hold it. [he claps a hand on Takebayashi's shoulder] There really is no saving you, is there? And here we went through all this trouble to give you the chance to become strong. Takebayashi: Strong? It looked like a group of people who were just scared. You and everyone else.
The reactions of Gakushuu and his father made this all the more amazing. Gakushuu almost completely lost it, becoming angrier than ever, when Takebayashi made it clear that he saw right through him. The Chairman looked livid at Takebayashi's defiance, and no longer seemed to be in control of the situation at all. One of his "pawns" had flouted the system, in front of the entire school. Who knows what long-term effect it will have?
And, as if all of that wasn't awesome enough, it turns out that Takebayashi's going to memorize all of the info the class needs to know about explosives, so that he can teach the rest of the class how to use them against Koro-sensei. Goddamn, Takebayashi.
Chapter 81: Is Karasuma-sensei really human?
Class E manages to outsmart Karasuma and Koro-sensei by leaving one squad in the pool so that it can't be captured by Koro-sensei, and another one to challenge Karasuma-sensei and lead him far away from the pool.
Chapter 84: Koro-sensei defeats Itona a third time. This time it's easier, and he even pulls off a new move he recently learned.
Nagisa openly contributing to the Koro-sensei mockumentary.
Chapter 94: Isogai boosts Itona into a 6-meter high jump that wins the boutaoshi game by sending Itona above the entire defensive team, straight to the very top of the pole.
Class E's entire strategy up until that point is also fantastic:
Half-toppling their own pole to pin Class A's first attack squad beneath it.
When E's attack squad is chased, running into the audience to evade since there's not rule against it.
Two E students everyone thought were knocked out were actually sneaking round the pitch to jump A's pole from behind.
E is able to leave only 2 people guarding their tilted pole and the five pinned A students by tricking the A students into thinking that Asano hasn't ordered them into doing anything yet and therefore they shouldn't get out and topple E's pole. Everyone else in Class E is therefore free to attack Class A.
Form the same chapter, Chairman Asano Curb-Stomping the four transfer students all at once. Shit just got real.
His training process? He got the crap beaten out of him the first day, did nothing but watch the second, and beat his sensei MULTIPLE TIMES—up till the point that Asano treated him as a child—without a single blow being landed on him on the third day. This guy's got balls made out of diamonds.
Note that he doesn't do this for the harm they caused the old man, despite the return of the Extreme Anger face, but for their attempting to make excuses for their behavior and not taking responsibility for their actions
Chapter 101: It looks like the class is trapped by the God of Death, until Okuda and Takebayashi just blow up the walls. Even the God of Death is amazed by it... which is not good, either.
Even if it's a bitch move by a traitor, Bitch-Sensei curbstomping all of Team B in chapter 104 with her Honey Trap tactics is badass.
Chapter 106: Karasuma's inhuman physical abilities. An explosive trap set behind a door? He kicks the door open then performs a backwards fall at the same speed of the blast with the door shielding him from damage. Six Doberman dogs trained to fire guns? He smiles and scares the living hell of him and they let him pass. Then he stops an iron beam flying at him with his bare-hands and grabs a crossbow just as it fires at him immediately afterwards. He may as well share Claire Stanfield's bloodline.
Chapter 114: Nagisa calmly tells his mother that he intends to stay in Class E and graduate from school... then cut his hair and pay back all of the money that she used to raise him. If she still tries to control him, he will "graduate" from her as well and walk away from the toxic relationship.
The entire school festival arc. Class E is pitted against Class A to see who can pull in more money for annual school festival. Class A pull out all the stops, opening up a cafe funded with corporate sponsorships that pay all of their food costs, as well as hiring famous comedians and idols to draw in crowds. Meanwhile, Class E mostly use what they can scrounge from the mountains to keep the cost of their dishes within school regulations, yet make them good enough to make the mountain trip worthwhile. And they manage to create a ramen so good that it pulls in crowds from all over Japan. They are forced to close on the second day of the three day festival due to running out of ingredients, and still manage to place third in the festival rankings. Even Class A acknowledged that Class E came close to beating them, and naturally, Chairman Asano was not happy about it all.
Chapter 122: The Final Question of the second term exams. It's an extremely difficult geometry problem that only Gakushuu and Karma have enough time and energy left to even attempt. Gakushuu sets up an elaborate geometric shape to brute force the problem, but he runs out of time before he can solve it. Karma takes the time to properly analyze the problem, and then utterly obliterates it in seconds.
Karma: Oh, come on. Even a kid in elementary school could get this!
The results of the second term exams: Everyone in Class E made the top 50. Even Terasaka. He placed 46th.
In the aftermath, Class A bows in apology to the headmaster, and states that if he is dissatisfied with their results, then he can move them to Class E, where they'll certainly be able to get better results. Given who they were saying that to, it is a show of guts the likes of which Class A has never demonstrated before.
Chapter 124: The headmaster, livid from the results of the second term exams, takes a shot at assassinating Koro-sensei. He puts Koro-sensei's teaching credentials to the test with laying out five workbooks that are holding live identical grenades, with four anti-sensei grenades and one normal. Koro-sensei must answer the questions bookmarked in four workbooks, and the headmaster will answer the remaining one, but they only have the very instant after opening the workbook to answer it and close the workbook before the grenade associated with it explodes. If there's ever a moment that shows how much of a Magnificent Bastard the headmaster is, it's this one.
And how does Koro-sensei handle this? The first workbook ends in failure for him, and everyone is being led to believe that this simple assassination method will actually do the trick... until Koro-sensei answers the following three workbooks flawlessly without a grenade going off. Koro-sensei then reveals that not only has he already memorized the workbooks involved beforehand, but he's also memorized every workbook in Japan. He only fumbled with the first because he lent that workbook to the students and so he forgot how to do that one. The headmaster's reaction to being left with the final workbook, which may or may not be the normal grenade, speaks for itself.
Chapter 126: When forced to answer the final workbook, Headmaster Asano is Defiant to the End, choosing to die trying to solve the problem rather than bow out and acknowledge Koro-sensei had beaten him. This leads into Koro-sensei being the Magnificent Bastard, as he uses his shedding to protect the headmaster from the grenade. When the headmaster asks why Koro-sensei didn't just use the shedding to protect himself from the first grenade, Koro-sensei reveals that he knew the headmaster would try to take his own life, and saved his shedding for just that moment.
Chapter 127: Class E is forced to perform a play for the rest of the school during lunchtime. To get revenge, they make the play as morbidly depressing as possible in order to cause the rest of the school to lose their appetites, ruining their lunch period.
Chapter 128: Creepy and as much of a shocking twist as it was, Kayano's revelation of having tentacles, having successfully kept them hidden from the entire class, and attempt to kill Koro-sensei with them is pretty awesome.
It's extra awesome on a meta-level when you realize, via the flashbacks and by going back through the series itself, that the author has been very very very very subtly hinting at this twist since early on, even going so far as to set up Nagisa as a giganticRed Herring by doing everything in his power to imply that Nagisa was the monster Shiro was talking about...all while Kayano was lurking imperceptibly in the background.
In essence, the author managed to perfectly assassinate every. Single. Reader. He got his weapon in close, hiding it in plain sight where none suspected, waited until the opportune moment when the reader's guard was down, and then... he struck! Precise, quick, and undoubtedly final. Truly a Moment of Awesome.
Chapters 131 and 132 has the fight between Koro-sensei and Kayano with tentacles. His plan against this opponent? To take a near-direct hit to his heart from one of her tentacles to lead her into a false sense of victory, to afford his students a chance to distract her from her bloodlust and help remove the tentacles before they consume all of her life energy. He also points out that this has a 50% chance of killing him, but he's so dedicated to ensuring that Class E graduates together and unscathed that he's willing to die for the cause.
And guess who steps up to the plate? The one best for the situation, Nagisa. His contribution? He takes a lesson from Bitch-sensei and forces a passionate kiss on Kayano, complete with a 15-hit combo! It's Big Damn Heroes and The Big Damn Kiss combined!
Chapter 133: Koro-sensei was the previous God of Death, the world's greatest assassin, two years ago. Take what prowess the God of Death that Class E encountered had, and apply that to Koro-sensei before now, maybe even amplify it. It's no wonder Class E got so good at assassination techniques in the span of a year!
Chapter 135: The God of Death knocks out Yanagisawa with nothing more than a burst of sound sent into his ear... made by striking a pane of reinforced glass with his bare hand.
Chapter 139: Remember the God of Death that Class E encountered, and his trick of shooting a tiny bullet at the aorta? The previous God of Death was able to do the same trick by flicking GRAINS OF SAND!
While the fact that they're fighting each other is a tearjerker, both Karma and Nagisa get these against each other. Nagisa almost instantly traps Karma in a triangular choke, showing off his skill, and Karma is able to move despite the hold, showing off his sheer strength.
The paintball war starts with a bang, showing off what the kids can really do. Takebayashi and Kataoka are taken out within the first second by snipers, and then Chiba and his guard are taken out by Kanzaki in a surprise attack. Irina is stunned at how good the kids have gotten.
Karma's reasoning for taking both of them out also indicates how much he respects their abilities - Takebayashi was planning on setting up a paint fountain bomb to barrage the other team and do multiple "kills" at once. Kataoka was targeted because Karma reasoned she would have the best shot of rallying teammembers on the field.
The climax of the paintball war is a Moment of Awesome for the Blue Team. In spite of their disadvantage in numbers and skill, they prove to be a match for the Red Team.
With Okuda's help, Isogai shoots down Mimura to prevent him from relaying information to Karma.
Isogai leads a suicide charge against Itona and Hayami. Yada manages to kill Hayami despite being inferior regarding their skill in marksmanship, while Maehara proves that bringing a knife to a gunfight is plausible when he kills Itona while Dual Wielding two of them.
Finally, Nagisa holds the highest kill count in the Blue Teamnote Tied with Hayami of Red Team. when he assassinates Nakamura, Terasaka, Yoshida, and Muramatsu in a second. Nagisa hid his presence under Karma and Mimura's noses because he was using Karasuma-sensei as a disguise. Unlike everyone else, he wasn't camouflaged for the mountain terrain. He was camouflaged to blend into Karasuma's camouflage. This allowed him to completely hide in plain sight, and jump the assault team when they least expected it.
And then there's the entire fight between Nagisa and Karma. All of it.
The entire plan to hijack government information on Koro-sensei in order to save him. It involves breaking into a space canter and hijacking a rocket to a space station to steal the information before it's sent back to earth and becomes even harder to get. And they do it, launching Karma and Nagisa into space.
Chapter 162: Funny as it may be, Korosensei proves his credentials as a master assassin when he takes group selfies of the class' more embarrassing moments over the year without a single one noticing or suspecting it.
Chapter 164: Koro-sensei is attacked by a laser, fired from a particle accelerator in orbit. Light travels at some 44,000 times his max speed. He still dodges.
Chapter 165: To prevent their interference in a joint international plan to kill Korosensei, the students are "taken into custody", and an official condescendingly explains the reasoning behind the plan. He ends it by saying that Korosensei's getting his "just deserts", as he was previously the God of Death, an assassin who's killed hundreds. Terasaka, despite being surrounded by dangerous mercenaries, promptly kicks him in the face and calls him out for being a smug, self-righteous asshole.
Chapter 166: Confronted by media reporters demanding to know why the school had gone along with Korosensei's presence and the training of Class E, Chairman Asano calmly says that he had felt Korosensei would be good for the children's education. When the reporters try to argue, he invites a few of them into his office for "exclusive details"... and uses his terrifying powers of manipulation to brainwash them into writing articles that cast the school in a positive light.
In the same chapter, Karasuma visits the kids and tells them off for trying to help Korosensei, detailing the obstacles that would have defeated them even if they hadn't been caught early. And by doing so, right under the noses of the kids' guards and the surveillance, he's giving them vital information on said obstacles, vastly increasing their chances of success.
Chapter 167: Bitch-sensei visits the kids and, to their collective surprise, kisses all of them—some rather deeper than others. When she leaves, several of the students find their mouths full of equipment and maps needed to make an escape. Bitch-sensei takes Sleight of Tongue and turns it Up to Eleven.
Chapter 168: Class E goes up against an army of highly skilled mercenaries. The mercenaries are on the ropes and running from them in fear.
Chapter 169: Class E against the boss of the mercenaries, someone that Karasuma claims to be three times stronger than him, and they beat him.
Chapter 174: Koro-sensei fires a MASSIVE beam at the second God of Death. No news yet on what happened to its intended target, but Yanagisawa gets knocked back by the blast, straight into the barrier of Anti-Sensei lasers, which disintegrates him like a bug due to having injected himself with tentacle cells.
Chapter 175: Koro-sensei shows that he learned from Aguri's death and he now has the knowledge and skills necessary to save Kayano's life when she's similarly impaled by stitching back together all of her cells. This is essentially Koro-sensei's My Greatest Second Chance, and he succeeds in glorious fashion.
Chapter 179: Class E ends up not accepting the 30 billion yen bounty for killing Koro-sensei. Instead, they only take the minimum needed to pay for living expenses, donate a portion of it to charity, use a portion to purchase the entire mountain their classroom is located on, and give the rest back to Karasuma and the government as thanks for taking care of them for the past year.
Chapter 180 has what is probably the most perfect Bookend as Nagisa steps forward as a unique teacher to a group of misfits, delinquents in his case, and Korosensei is even briefly seen from the back by one of them after Nagisa proves that even seven years later, and with no height gain, he's damn well still a badass and 3-E member...and Korosensei's final legacy.
Nagisa: I hope you can! Kill me, that is. You have until graduation!
The Class E system finally being abolished at the end of the series, thanks to news of the system spreading to the outside and the public outrage that ensued. Now future generations of students don't have to endure such mental torture.