The Joker's Robbery of Gotham Merchants Bank was the first sighted crime to be openly conducted by the Joker, as well as the first confirmed sighting of the clown prince of crime.
Prelude
Roman Sionis, also known as the crime lord Black Mask, owned the Gotham Merchants Bank as a front for his money laundering activities. At the peak of his power, he noticed the arrival and suspicious actions of a then-small time criminal known as the Joker, stealing chemicals from his steel mill and harassing his girlfriend Tiffany. Eventually, the Joker got the drop on him by invading one of his safehouses, abducting Tiffany, and then lying in wait for Sionis to arrive. After killing a decoy, he then captured Roman Sionis alive after subduing him in a failed ambush on him, as he needed him alive in order to access the Gotham Merchants Bank for the planned robbery on it, as it had safeguards that only Sionis himself could disable, including biometrics.
Execution
On Christmas Eve, while disguised as Black Mask, Joker and his followers proceeded to steal an ambulance and, after arriving at the bank, proceeded to ram it through a wall to the vault in reverse. Joker then took the staff hostage, and proceeded to brutally murder about two-dozen employees by beating them with TVs and the various Christmas decorations in the lobby. He then took the remaining employee, a female bank manager, hostage and proceeded to inject her with Joker venom, enough to ensure a slow, painful death. He then accessed the vault, proceeding to steal a huge sum of money, some of which are even kept loose, and finally rigged the vault to explode with various remote-controlled explosives.
Batman, deducing the Joker's planned robbery from investigating the servers at the GCPD, arrived at the Gotham Merchants Bank just as the bank robbery was at its conclusion. He then broke into the main vault just as they were about to load the last bag onto the ambulance. Joker, while still acting like Black Mask, then accused Batman of just waltzing into his bank, with Batman informing him that he's only there to apprehend the Joker. Joker then denies any knowledge of him, also briefly "asking" the manager whether she knows who the Joker is, and then eventually asking the real Sionis, who proceeded to expose him as an imposter by threatening him for trying to steal his money, causing the Joker to beat him viciously for "not playing along." After having his men load the fallen crime lord into the ambulance, Joker then proceeds to unmask himself in front of Batman and confirm he is indeed the Joker. Batman then deduced that the Joker was also responsible for everything that happened that night, including hiring the eight assassins against him, and overall running Black Mask's operations, with Joker correcting him that they are his operations now, with his men replacing the black masks with clown masks to demonstrate their affiliation change. Joker then proceeded to shoot the hero with his gun three times in the chest while laughing maniacally, also revealing that his earlier actions against Batman were just a "teeny little distraction" compared to what he has in store for Gotham. He then threw the terminally poisoned manager at Batman and made his leave, also taunting him about the sounds of "eight tiny reindeer" before having the ambulance drive off, leaving two of his men behind. He then set off the explosives, forcing the Dark Knight to grab the manager and duck back into the lobby, the explosion being strong enough to force the safe door off and block the main door. Ultimately, the manager had succumbed to the poison by the time Batman came to.
Aftermath
The Joker then sent three goons to the bank to make sure he's dead. He then expressed shock that Batman could still be alive despite shooting him three times in the chest as well as blowing up the vault when they reported that they can't find his body, and ordered for them to kill him, threatening that they'll die by Joker's hand if they don't. Batman then proceeded to take them out, also learning that the Joker, after taking over Sionis' operations, had killed nearly a quarter of Sionis' men due to their refusing to acknowledge him as leader over Black Mask, and that he was most likely at the Steel Mill.
Ultimately, this action was what cemented The Joker as being the most powerful crime lord in Gotham, as well as the start of his infamous terror spree in the city for several years to come.