Denji has engaged in combat with more demons than one can count during Chainsaw Man. Even though he is a human-devil hybrid, lives with the Blood Fiend, and has collaborated with numerous other fiends in Public Safety, not every encounter has ended in death. The airhead protagonist frequently has no awareness of the evils all around him, many of which are right in front of him.
It's not unexpected that so many devils have been mentioned in passing in a reality where they are the primary danger to humanity. The Primal Fears are thought to be the most potent of all the monsters, from the Tomato Devil to the Four Horsemen. There is one devil that tops them all, though.
Human Fears Give Birth to Chainsaw Man's Devils
The Darkness Devil or the Gun Devil are examples of devils who are born in hell and are made from human anxieties. They get their strength from fear as a result. The demon associated with a phobia grows stronger the longer it has lived or the more popular it is because they feed on human fears. Because devils embody the terror that their names provoke, they will keep reincarnating and gaining power as long as the thought of them inspires fear.
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 Most Powerful 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.
Pochita is the only devil who could possibly compete with this Primal Fear. Without Chainsaw Man's ability to completely erase devils, devils would either lose all of their strength or be forced to find another medium from which to draw their power. However, this assumes that Pochita will be able to approach an entity with as much knowledge, experience, and strength as the Fear Devil.