While the title of the manga is Chainsaw man, at first glance it does not seem like he would be the strongest person in his own show. With existences such as a Control Devil, Gun Devil, and even a Darkness Devil, it seems unlikely for a Devil with the appearance of a Chainsaw to take on the others.
However, as the series goes on and we see Denji taking on one adversary after the next, we can’t help but notice that there’s more than what meets the eye. From the start, the devil always showed extraordinary strength, healing, speed durability, and a lot of scope for improvisation.
Coupled with Denji’s relentlessness, it seemed like a match made in heaven. This still does not explain why every Devil either admires or fears the Chainsaw Devil so much. Even the Darkness devil, an embodiment of fear itself seems to seek it out.
We finally learn towards the second half of the series that the Chainsaw Devil has the ability to erase other Devils from existence and is the only one that can do so. It can make all their achievements or purpose itself meaningless and is known as the Hero of Hell.
Any devil that is eaten by the Chainsaw Devil is erased and memories of them are also wiped out. It is said in the manga that Nazis, World War 2, Nuclear weapons, and several others have already been eaten and hence erased by the Chainsaw Devil.
The Angel Devil also says that Devils can never indeed die as long as people remember their name and that they eventually end up reviving in hell. On the other hand, Devils who die in hell get revived on Earth but with their only memory being a “BROOM” sound, similar to that of a chainsaw getting started.
This further explains how the fearsome power of the Chainsaw Devil since it does not let other devils revive and completely nullify their existence.