The Laughing Fish is the greatest Joker story ever told, full stop. That story was written by Steve Englehart on his run in Detective Comics from #469-476 with Marshall Rogers on art duties; arguably the greatest run on the character in its 95-year history. So much of that eight-issue sequence has seeped into the character's essential fiber that its remarkable that Englehart's name isn't more widely-known. Of course, part of why it's not is simply the way Hollywood and TV function, which is highlighted in this specific case. When you see the credits for this episode, you'll see that it's "Written by Paul Dini." Of course, the story is Englehart's and they even use segments of the dialogue ("Jokerburgers!") from Englehart's story. I pointed that out to him when I ran into him at a comic show about a year after the episode aired. He nodded and said that he had been paid a substantial "service payment" in lieu of a credit. Since he was under a work-for-hire contract when he'd written the story, like the vast majority of comic writers and artists of that time, he probably didn't have much choice. Just the same, it still rankles me every time I think about it.
Thankfully, Dini not only used the dialogue but stuck to the main elements of Englehart's story, as well. In the years since The Batman had returned to being the Darknight Detective, rather than "good ol' Batman", the Joker had still lagged behind the change in tone. He was still the Clown Prince of Crime, with the emphasis on the "clown" part. A bit more serious and threatening, but still more bizarre than he was dangerous. Englehart changed all that. In The Laughing Fish, The Joker is a homicidal maniac whose actions can't be predicted, making him terrifying to anyone and everyone. Dini kept that perspective, using the laughing gas made of Jokervenom that the character would then use forever after, in which victims are killed and die with a rictus grin on their face, reminiscent of the villain himself.
As BTAS was still aimed primarily at children, Dini couldn't show The Joker's intended victims actually dying (Batman would rescue them and be rescued himself with last-second antivenom doses), but it was still easily the most eerie of situations in the series' history. The character is also at his most menacing, as he regularly threatens the copyright official (G. Carl Francis; played by George Dzundza) with death, which isn't something that regularly happens in the series, other than the usual threats to our hero. The presence of Harley Quinn does spoil the presentation a bit, as she's an intentionally goofy character who doesn't mesh very well with this more menacing version of The Joker.
This is also one of the episodes where Harvey Bullock (most frequently played by Robert Costanzo) functions less like a corrupt buffoon and more like an actual hard-boiled cop who doesn't want to be upstaged by the guy in the bat costume. It's Harvey, of course, who first finds The Joker's hideout and tried to finish the job of apprehending the villain. It's a story that's more grounded in the noirish elements that the series was rooted in than the previous episodes. Since it also features The Batman's greatest opponent, it's hard to look at this as anything but among the very best three or four episodes ever produced, even if the plot itself is relatively straightforward in a "villain of the week" style that has often made superhero comics even more formulaic than they typically are.
Those noirish elements persist right through the conclusion, with the villain disappearing under the water, The Batman doubting that he's actually gone, and the shark devouring the floating card as the dénouement. This is what the series should have been even more often and it's necessary to see it to really understand how great Timm and Dini's (and Englehart's!) efforts were. My one complaint is the title card. It was a departure from the neo-Gothic art style used in most of the rest (until season 4) which were mini-representations of the episode that resembled 1930s movie posters. Instead, this one had a very simple font and no real character at all, which is disappointing.