Many fans consider Bane to be the Batman enemy who best combines brains and brawn, but Scarecrow may actually be the Dark Knight's smartest and strongest foe. Jonathan Crane is the master of fear, with his fear toxin that can cause anyone to go into hysterical fits of terror, but he has one other ability that is much more horrifying.
Almost all of Batman's enemies have some manner of horror-based element to them, but Jonathan Crane aka the Scarecrow bases his entire identity around horror. Using his fear toxin he can cause people to sometimes even literally die from fright. The only downside that Crane has is the fact he's not exactly the best hand-to-hand fighter. Some depictions of him have given him the crane martial arts style, but by and large, Scarecrow is not a capable fighter or a physical threat. However, that all changed when Crane unleashed a terrifying new power.
In 2004's Batman: As the Crow Flies by Judd Winick and Dustin Nguyen, the Penguin gathers together a crime family to rule Gotham City, and to do this he employs Johnathan Crane to provide a fear toxin to keep his underlings in line. To ensure this is done, Penguin gives Scarecrow an assistant to help with his research. While all of this is happening a strange monster has been hunting down Penguin's men and slaughtering them. This fearsome monster is 10 feet tall, able to move with remarkable speed, has super strength, and—most terrifying of all—is able to release fear toxin from its breath. This makes the creature one of the most physically intimidating enemies in Batman's rogue's gallery. Even though Batman has faced Hulk-like enemies before, the fear toxin that this beast unleashes can bypass all of Batman's anti-fear toxin technology, including anti-toxins and gas masks. This makes him significantly more dangerous than what Batman is used to handling. However, as it turns out, this monster is no mere mindless beast, but actually none other than Scarecrow himself.
Scarecrow Gains Hulk Powers To Become Batman's Most Dangerous Enemy
At the end of the story, it's revealed that the assistant Penguin appointed to Scarecrow has dosed Crane with something that causes him to transform when under stress or during feelings of danger. During this time Scarecrow becomes a near-mindless beast, hunting down those who are fearful. Penguin does this to have Scarecrow kill off his underlings, so Penguin can inherit the entire criminal empire without having to share the profits. However, this may not be the best trade-off since Scarecrow gains an incredible new ability that makes him far more dangerous than ever before.
Batman is used to dealing with dangerous enemies, but usually, if the enemy is a chemical expert or a master strategist, they don't tend to also be incredible fighters. Bane is one of the lone exceptions, which is what made him so dangerous. Riddler is one of the most intelligent enemies Batman has ever faced, but he's not a fighter. So the idea of a cerebral villain like Jonathan Crane undergoing a Hulk-like transformation makes Scarecrow one of the most dangerous enemies Batman has ever faced.