Throughout the course of Chainsaw Man, Denji has come to blows with more demons than can be counted. Not all meetings have been fatal -- after all, he is a human-devil hybrid himself, lives with the Blood Fiend and has worked with many other fiends in Public Safety. Often, the airhead protagonist is ignorant of the devils around him, with many right under his nose.
In a universe where the main threat to humanity is devils, it's not surprising that so many have been mentioned in passing. From the Tomato Devil to the Four Horsemen, all vary in strength, with the Primal Fears known to be the most powerful. However, there is one devil who surpasses even them.
Chainsaw Man's Devils Are Born From Human Fears
Devils are born in hell and created from the fears of humanity, hence the Darkness Devil or the Gun Devil. As such, they draw their power from terror. Since they feed upon humanity's fears, the longer a fear has existed or the more popular it is, the devil correlated with that concept swells increases in power. Devils are the very embodiment of the fear that their names represent, so as long as the concept instills fear, devils will continue to reincarnate and amass power.
The concept of a devil itself doesn’t have to be scary on its own; rather, its power relies on how frightening it is in the minds of humans. For example, cars are not feared alone, but it is stated that the Car Devil is strong because humans often think of car crashes and other related motor-vehicle accidents, feeding its power.
By this logic, the strongest devil would be the oldest and scariest of them all, whether the concept it's based on is tangible or abstract. Primal Fears are devils that have existed since the dawn of time and embody some of the deepest anxieties of the human psyche. The only devils that could rival their strength are the Four Horsemen, so CSM's most powerful foe would be a Primal Devil -- likely the oldest one of the infamous group.
The Fear Devil Is the Strongest in Chainsaw Man
The devil that is undoubtedly the oldest would be the one emulating the fear of fear itself. Before humanity even knew what they were afraid of, they felt fear as an emotion. This is a concept that has always existed within the human mind, so this devil would have been collecting power from day one. On top of this, because devils feed off fear, a Fear Devil would accumulate the power of every single devil in existence.
Every new concept that someone becomes afraid of or any constant fear that maintains its state would feed the Fear Devil's strength. This is a devil that, in theory, could extract power from every human alive, from the minute they're born to the end of their lives. No matter the popularity of the fear, from sea cucumbers to war or famine, these anxieties would make the Fear Devil an immensely powerful threat to humanity.
The only devil that could possibly contend with this Primal Fear would be Pochita. Chainsaw Man's ability to erase devils altogether could strike out fear from humanity, but without this, devils would either lose all their strength or be forced to find another medium from which to draw their power. However, this is assuming that Pochita would even be able to get close enough to an entity with as much gathered knowledge, experience and strength as the Fear Devil.