Catwoman: This is a bit more complicated because Catwoman villain status is all over the place. She's kind of a tweener in that regard. My favorite story involving her is Batman: Year One, but she's actually my least favorite part of that book. If you want a book just focused on Selina Kyle and her life, Ed Brubaker-Darwyn Cooke/Cameron Stewart Catwoman comics are out there, please read them. Pre-Crisis Catwoman/Batman romance? The wonderful Alan Brennert written story from Brave and the Bold #197 "The Autobiography of Bruce Wayne". Just a quick dose of Catwoman stealing shit and being sexy? Selena's Big Score by Darwyn Cooke or the first two issues of Batman Inc vol.1 "Mr Unknown is Dead" by Grant Morrison and Yanick Paquette. Lot of great options out there, none of them involve Ann Nocenti.
Harley Quinn: "Mad Love" - Dr. Harleen Quinzel, on the other hand, is much simpler to nail down what the best story is. It's "Mad Love" by Paul Dini and Bruce Timm, my absolute favorite Batman story of all-time. Accept no substitutes.
Poison Ivy: "Harley and Ivy" - I find Pamela's brand of eco-terrorism more palatable when she's teamed up with Harley, and this is the peak of that team-up. Paul Dini and Bruce Timm once again, in which the story doesn't have any of the psychological depth of "Mad Love", and is
merely delightful pop comics done by two top-tier storytellers. Looooot of fan service here too, I mean a WHOLE lot of fan service. But also quite funny, like when they teamed up to steal a super-rare priceless plant Pamela wanted and Batman dropped in.
Mr. Freeze: Ok, I'm doing some cheating here. The correct answer here is "Watch 'Heart of Ice' and 'Deep Freeze' from the Batman Animated Series", and I'm not gonna argue with it. After you do that, than you can enjoy Paul Dini and Glen Murakami's "White Christmas" from the Batman Adventures Holiday Special #1. Its a small tale, but its done by two of the big leaders behind the great TV show and
its easy to miss what an elegant and intelligently designed story it is.
Mad Hatter - "Unresolved" (Gotham Central #19-22)- There's a few Hatter stories I could have picked, but my favorite one he was heavily involved with was Unresolved from Ed Brubaker and Michael Lark. Mostly it allows me to
beg you to read Gotham Central, because like Secret Six it was a great book that more people should have bought.
Red Hood - Cheating again, because there's the original story "Under the REd Hood" Judd Winick and my boy Doug Mahnke did, and you can read that if you like. BUT they recently did a movie version that's actually even better, with Winick on scripting duties and reworking some of the problematic elements of the original into something thrilling and often dramatically satisfying. So if you want the best, watch the movie!
Killer Croc - The only Croc comics I've found worth a damn are the original ones, written by Gerry Conway and drawn by Gene Colan, Don Newton, and Curt Swan. Here, instead of the big dumb muscle he's often portrayed as later, he's actually a pretty smart guy you just wouldn't want to mess with. Batman #357-359, or maybe Detective Comics #525. Alternatively, watch "Vendetta" form the Batman: Animated Series, a cool little noir thriller with a hint of horror movie elements. Croc's actually pretty smart in this one too, although he's now more famous for "I THREW A ROCK AT EM" from "Almost Got 'Em"
Man-Bat - Two options here, you can with the original well-regarded Man-Bat story from Detective Comics #400, or my preferred "Batman and Son" from Morrison and Kubert. The latter doesn't really star Langstrom, but frankly I always thought he was a boring character; the real interest came from the visual juxtaposition of seeing Batman vs Man-Bat, right? Where Batman and Son has that in spades, in a cleverly staged set piece where Batman fights an army of ninja Man-bat commandos(that's right) in an pop comic art museum.
Talia Al Ghul - I want to say the entire Levithan mega-arc from the last leg of Morrison's Batman epic, but again, you need to read from the beginning to get the full effect on how effective her supervillainy is and the total characterization Grant was working with. For a taste, once again, "Batman and Son" will start you down the path. If you just want a quick dose of Talia though, you can do a LOT worse than the Batman-Spider-Man crossover, "New Age Dawning" by J.M. DeMatteis and Graham Nolan/Karl Kessel. Both this and the other Marvel-style Spidey/Bats team-up issues are rather good, cuz they have GREAT art and a writer who really understands the universes. This is my preferred one, its darker Batman tone allows DeMatteis space to delve into characters. And if anyone read his work before, you know DeMatteis is great with understanding characters, and mines the unique crossover story for all its worth. For example, the great scene where Vanessa Fisk and Talia Al Ghul bond over being in love with men who are also monstrous.
Hugo Strange: "Prey" (Legends of the Dark Knight #11-15) - Strange started off as a mad scientist with giant men and spooky castles and all that, but he slowly got toned down a bit, first into a more modern setting in his great appearance in the aforementioned Engelhart-Rogers-Austin run, and then later here where's more of a demented psychiatrist believing he has the key to crack the Bat. I really like this story; its structured a lot like The Dark Knight in that its a dense crime drama mixed with psychological depth and superheroic elements. Paul Gulacy really shines in the action scenes, his Masters of Kung Fu background showing in his martial arts posing and staging.
The Ventriloquist/Scarface - Once again, you got the well-regarded original stories by the great 90s Batman team of Alan Grant and Norm Breyfogle. Detective Comics #583-584 is what you want. He always features pretty heavily in the year long, underrated, David Brothers-approved "City of Crime" epic, Detective Comics #801-814.
But my personal favorite story involving Scarface/The Ventriloquist is from the Animated series, "Read My Lips". Top 5 episode in the series, its got everything you want from the show all: a great villain who’s as threatening as he’s psychosally screwed up, sharp witty dialog, smooth animation, another fantastic score, moody visuals to perfectly illustrate its 1930s gangster cinema atmosphere. Love it.
Uh did I forget anybody else
How can I control myself when there's so much to buy and read?