Few comic readers would dispute that, as far as rogues galleries go, Batman has one of the most dynamic assortments of adversaries compared to his superhero brethren. Still, as popular as decade-old villains like the Joker or Catwoman remain, Batman's rogues gallery would've never become what it was without creative innovation, so the continued development of original villains should be encouraged.
To that end, it's worth taking a look at some episodes of the acclaimed Batman: The Animated Series (B:TAS) which featured original antagonists conceived entirely for the show.
10 "The Forgotten" Features Boss Biggis
"The Forgotten" is one of the most unusual episodes of B:TAS - a Cool Hand Luke homage and an experiment. It primarily features Batman out-of-costume, all tied together by some of the most overt political messaging in the show, tackling real-life problems which gallingly persist even today (in this case, homelessness and human trafficking). The episode's villain, sloven slave-driver Boss Biggis, is effectively one-note. As a personification of greed, he's not a character with much purpose beyond this one-off, but he serves out that purpose effectively.
9 "Day Of The Samurai" Features Kyodai Ken
Across most continuities, Bruce Wayne's globe-trotting training in prep for his vigilante career almost includes training in Eastern Martial Arts - B:TAS explores this element of the titular character's backstory across the interconnected tale, "Night of the Ninja" & "Day of the Samurai." The latter is the superior of the two, featuring Kyodai Ken, a rival of Bruce from his training days, returning to Japan and Bruce being lured in after him. The episode's Japanese setting is a refreshing change of scenery from the series' usual Gotham City backdrop, while the final duel between Batman & Kyodai Ken, where the Dark Knight for once has a foe who can fight him mano a mano, makes for one of the strongest action scenes in the series.
8 "His Silicon Soul" Features H.A.R.D.A.C.
Like "Day of the Samurai," "His Silicon Soul" is another overt sequel episode superior to the original. "Heart Of Steel" is a fine episode on its own terms, but "His Silicon Soul," featuring the return of A.I. H.A.R.D.A.C. and a robotic duplicate of Batman convinced he's the real thing, packs more emotional storytelling into a tighter package.
Though goofier than the average B:TAS episode, what anchors the episode is Kevin Conroy, who gives one of his greatest Batman performances as the duplicate Dark Knight; his acting makes Batman closing question if the robot had a soul all the more potent.
7 "See No Evil" Features Lloyd Ventrix
"See No Evil" has one of the effective, high-concept pitches in B:TAS: Batman versus the Invisible Man. Now, the episode's antagonist, Lloyd Ventrix, isn't a direct adaptation of HG Wells' character (for one, Ventrix is not the inventor of the disguise granting invisibility, but merely a thief), but the influence of that story on the episode is blatant. The episode also manages to walk a fine line between making Ventrix human but not overly sympathetic - he begins using the invisibility cloak to visit his estranged daughter in the guise of her imaginary friend "Mojo," but he's still a thief, and that tendency escalates when he plots to abduct his daughter as well.
6 "Zatanna" Features Montague Kane
"Zatanna," featuring the titular sorceress and detailing her history with Batman, is one of the few times that B:TAS waded into the wider DC Universe or the supernatural. Originally, however, the episode planned to take this even further by featuring Hawkman adversary the Gentleman Ghost - producer Alan Burnett, wary of overtly extraordinary stories, shot that down. Thus, the episode's antagonist was rewritten into the entirely human con-man Montague Kane (modeled on Orson Welles, named after his most famous role), the only merely serviceable part of an otherwise excellent episode.
5 "Baby Doll" Features Mary Dahl
The titular villain of "Baby Doll" is actress Mary Dahl, afflicted with both systemic hypoplasia (a fictional disease that prevents her from aging) and a face resembling Elmyra Duff. After a disastrous turn at dramatic acting, Dahl begins kidnapping her cast-mates from a sitcom she once starred in, "Baby Doll." The episode is initially quite lighthearted, but the climax - when Dahl breaks down in a house of mirrors after seeing a distorted image of what she'd look like if she'd grown normally – injects the episode and character with unexpected pathos.
4 "Harley's Holiday" Features Harley Quinn
B:TAS introduced Harley Quinn to both the world of DC Comics and pop culture. And there's plenty of great episodes featuring her which could appear on this list, but ultimately, it has to be "Harley's Holiday," for it's this episode that's proved most critical for her development.
For one, it's the first episode of hers where she's the unequivocal star. Batman himself is a mere supporting player, while the Joker is entirely absent, the clearest indicator that Harley had grown beyond her intended role as support for the Clown Prince of Crime. Plus, putting her in the role of protagonist foreshadows her modern solo and anti-hero phase.
3 "It's Never Too Late" Features Arnold Stromwell
B:TAS took much stylistic influence from Film Noir - "It's Never Too Late" is an episode where that influence is most apparent, from the gangster-focused story to black-and-white tinged flashback sequences. There's also the brilliant role reversal at the episode's core - Batman is merely a supporting player, while aging gangster Arnold Stromwell, who should by all rights be the villain, is the protagonist. Across the episode, Stromwell reckons with the consequences of his actions, from his brother having lost a leg in a train accident to his son's drug addiction, culminating in an act that proves the episode's title true.
2 "Beware The Gray Ghost" Features Mad Bomber
"Beware The Gray Ghost" is an unmatched tribute to actor Adam West and the Batman series in which he headlined; West guest-stars as Simon Trent, the out-of-work actor who once played Bruce Wayne's childhood film serial hero The Gray Ghost. When crimes reminiscent of a "Gray Ghost" episode begin happening around Gotham, Batman tracks down Trent for help in obtaining a "Gray Ghost" film reel to solve the case.
The episode is a brilliant meta-textual work; Bruce being inspired by the Gray Ghost when crafting his Batman persona reflects how it was West's Batman which ensured the title character became popular enough to endure for decades on. Likewise, the episode's villain, the Mad Bomber, turns out to toy shop owner and "Gray Ghost" fan Ted Dymer (voiced by series co-creator Bruce Timm), reflecting how many fans' passions can coil into something more nefarious.
1 "Appointment In Crime Alley" Features Roland Daggett
Whenever B:TAS required an antagonist in the form of a nefarious but "legitimate" businessman, Roland Daggett, hastily renamed from Batman Returns' Max Shreck, tended to fill the role. Daggett debuted in "Feat of Clay," being the inventor of the chemical concoction that transformed Matt Hagen into Clayface, but "Appointment In Crime Alley" is an even better showing for Daggett (and, just barely, an even better episode overall). For one, there's no super-villain involved in the proceedings to overshadow Daggett's more mundane evil (here, a gentrification scheme). There's also the fact he escapes justice while the poverty in crime alley continues, meaning the episode ends far closer to bitter reality than most others.