A fight between Goku of Dragon Ball Z and Koro-sensei of Assassination Classroom would be one of the most unique battles between anime and manga characters imaginable. When it comes to the power levels, Goku is often the bar to measure against. With hundreds of chapters and dozens of villains under his belt, Goku's a force to be reckoned with. And as far as character debuts go, Koro-sensei starts things off with a bang and never lets up—the yellow tentacled teacher prides himself on being nigh unkillable. What might happen, then, if the version of Earth that Koro-sensei ended up on was being defended by Goku? Could Goku protect the planet, or might he finally meet his match?
To evaluate how these two might fair if they were to square off, it's important to take a solid look at their canonical abilities and feats. This can help to establish a baseline power level for each, and make comparison a bit easier. Another factor to consider is experience: what kind of enemies have each faced before, and what useful knowledge could these past fights provide that could sway this matchup? With that in mind, it's time to evaluate the combatants, and keep spoilers to a minimum.
Goku (Dragon Ball Super era)
Since Goku has had such a long career, it's important to establish exactly when his abilities come from. The fairest option is to take the most recent point of continuity that most are familiar with. Thus, Koro-sensei will have the pleasure of going up against a post-Tournament of Power, Dragon Ball Super era Goku, giving Goku essentially the full history of his accomplishments. Goku, at this stage, has not only achieved Super Saiyan God and Super Saiyan Blue forms, but also the Ultra Instinct form, which even the gods can't easily master. Goku has most recently battled the strongest beings from across the multiverse and come out on top, although certainly not through his efforts alone. He has a wide range of abilities at his disposal, including instant transmission (essentially teleportation), energy attacks like his Kamehameha wave or the Spirit Bomb, and exceptional skill at traditional martial arts techniques. While speed isn't usually touted as one of Goku's strong suits, he's still incredibly fast, even in his base form. Goku also benefits from his Saiyan biology, in that every near-death encounter drives him to develop new abilities and achieve new levels of power. He has died on several occasions, but he's always able to come back eventually, and uses that time in the afterlife to train with the greatest warriors throughout history.
Goku's battles with major villains throughout the years also carry with them some valuable experiences that could be put towards a fight with Koro-sensei. Cell's regenerative abilities, for example, bear some similarities to those of Koro-sensei, so Goku likely wouldn't be discouraged or surprised by that. Goku has also battled enemies with non-humanoid body types, like demons and aliens, and those with exceptional body elasticity, like Majin Buu or Piccolo. Goku's battles with Frieza and Lord Beerus also show that he has no problem with the vacuum of space and other exceptionally deadly environments, and can fight just as easily in midair as on the ground.
If Goku has one major weakness, it's his desire to fight only the strongest, and force his enemies to the peak of their power. He's known to hold back, rather than jumping immediately to Super Saiyan or higher when confronted, and will often try to match his form to the power level of his enemy. This sometimes backfires for him, as it pushes the enemy to new heights that he can't deal with at that moment. Characters like Cell and Buu were able to take advantage of that, and Koro-sensei would definitely recognize this weakness in him.
Koro-sensei (Assassination Classroom)
A strange, octopus-like being who's allegedly from space, Koro-sensei worked out a deal with the leadership of Earth: he'd take over a class of high school students and train them for one year. If the class can kill him by then, the Earth survives, and if not, he destroys the Earth. Koro-sensei is shown to be exceptionally fast, traveling to other continents for lunch and even creating afterimage illusions. He possesses some excellent regenerative abilities as well, to the point where he's willing to cut off limbs as a handicap when fighting his students. Standard weapons are completely unable to harm him, and only the "anti-sensei" weapons he provides the class are capable of dealing even temporary damage. He can even become invisible, although that doesn't extend to any gear or clothing. And of course, his Ultimate Defense technique reduces him to an immobile sphere for 24 hours, but also renders him indestructible.
While his offensive abilities are far less focused on, the extreme speed with which his limbs can move make attacks nearly impossible to avoid. His smiley face head also means that his moves are harder to anticipate, as despite color and pattern changes, it doesn't give away much to those who aren't familiar with their meaning. Throughout the series, Koro-sensei goes up against other beings similar to himself and takes on a plethora of Earth's greatest assassins, defeating techniques from guns and missiles to poison to martial arts. In fact, it seems as if training students to defeat him specifically is essentially the only way for the Earth to have a chance at success at all.
For Koro-sensei, there are a number of weaknesses: being submerged in water, easily distracted by beautiful women, and of course the "anti-sensei" weapons the students are provided. One trait that might function as a weakness, although it normally wouldn't be considered to be one, is his desire to turn situations into teachable moments. While Koro-sensei certainly possesses the ability to get serious and fight, an opponent that lacks evil motives would likely be greeted with advice rather than serious attempts at destruction.
Who Would Be The Victor?
A battle between Goku and Koro-sensei would certainly be a fight worth watching, and one that both characters would surely enjoy. Goku's approach to fighting is to always seek out a stronger opponent, one who can force him to reach greater heights, but he also doesn't use his strength to bulldoze those weaker than him. While it probably wouldn't take long for Goku to start breaking out his more powerful transformations, there's another aspect to his behavior and fighting style that would catch Koro-sensei's attention. Though Goku can be rather thick when it comes to understanding basic things, he's a fast learner when it comes to combat. As Goku learns more about his opponent, observing the regeneration and speed firsthand, his tactics would undoubtedly change.
Koro-sensei would be thrilled by this, and begin taking the opportunity to teach Goku how to engage with him. In many ways, the relationship might begin to resemble that between Goku and Whis, a strange and powerful being impressed by Goku and opting to teach him more, if only to see just how high he can reach. With techniques like Instant Transmission and Ultra Instinct at Goku's disposal, Koro-sensei's speed would become less of an issue. Goku might even come out of the fight stronger than when he started, as Koro-sensei helps him to master Ultra Instinct. While it's not clear exactly how well Koro-sensei can stand up to energy-based attacks, he does have access to some Ki abilities that might make him more able to counter a Kamehameha than one might expect.
In the end, Goku seems most likely to win, if only because of Koro-sensei's eagerness to coach his opponents. Goku's experience with alien invaders and mutated creatures with absurd regeneration would provide him with an edge to begin with, while Koro-sensei mostly has experience with ordinary humans. Even if Koro-sensei were forced to activate his Ultimate Defense, it would place him in exactly the dangerous spot he's always feared: helpless to resist as someone transported him into space. Goku wouldn't kill Koro-sensei unless left with no choice, and they should have access to the dragon balls as well, offering solutions that Koro-sensei would not usually have access to. Perhaps the pair could find a way to coexist and train together after the fight?