Having compiled 30 mammorable moments from DC Comics History from my other blog and from other sources, here's twenty more, rounding my count off to 50, as I'm not quite motivated enough to make it to 75.
(31) "The Million Dollar Debut of Batgirl," DETECTIVE #359. Ordinarily I'd avoid origin-stories as being "memorable moments" because, with a few exceptions, they're just devices to start the protagonist on the road to significant accomplishments. But I do think the Fox-Infantino Batgirl tale, in addition to being a good yarn that lays out the character's motivations pretty well, is historically significant thanks to the fact that Batgirl's creation was mandated by the 1960s TV show. Not that DC probably wouldn't have evolved a new Batgirl idea on their own someday, but it does mean that the show's TV producers ended up doing the comics some long-term good, whatever slings and arrows the campy show might've brought about in the short term.
(32) I was going to call this entry "Supergirl reveals herself," only to find that there actually WAS a Jerry Siegel story called "The Day Supergirl Revealed Herself," from ACTION #265 (1960), in which the title character got amnesia and showed herself in public, thus imperilling Superman's plans to keep her hidden in his fortress dung-- er, keep her in reserve as a "secret weapon." At any rate, I'm actually referencing a story roughly two years later, in ACTION #285, in which Superman reveals Supergirl's existence to the world at large.
(33) You have to love the title of the story in BRAVE AND BOLD #63 for its stupefying corniness alone: "Revolt of the Super-Chicks!" But historically speaking, it's the first time DC put the spotlight on two unrelated heroines teaming up even for one story, with no superguys to spoil the hen party.
(4) Not wanting to leave out the villains' accomplishments, I open with the female Mist's rape of Jack Knight in STARMAN...
(5) ...And then raise with the villainy of uncontrolled birth seen in GREEN LANTERN #81, in which the sterile Mother Juna (note the reference to Roman "Juno," and maybe to the Hindu term "yoni" as well) nearly destroys her planet Malthus by having so many children, Green Lantern and Green Arrow don't know what to do.
(6) Poison Ivy's another DC character whose debut rates mention. Her main object in BATMAN #181 is to dethrone three other Gotham crime-queens (who never appeared before and have only recently been revived). Just like a woman; she just has to be the center of attention! But she did launch a growing trend for female villains in the Bat-books, which is rather impressive given that aside from Catwoman BATMAN hardly had any female villains in the previous twentysomething years.
(7) Catwoman's the perfect example of a character who isn't really much to write home about in her first few stories, but who grows as writers build upon her. Not that all of the additions were stellar: Bill Finger's origin for his creation made her an amnesiac whose id got out of control. But in the early 1950s, just before the Comics Code arguably exiled Catwoman from the printed page for roughly ten years, she was briefly returned to full villainous status. In DETECTIVE COMICS #203 she gets pissed because she thinks Batman's been boasting about all the times he defeated her, and summarily goes back to crime-- and of course, the thrill of dueling with the Big Bat.
(8) The adult version of Lana Lang worms her way into the world of Superman and his unofficial "girlfriend" Lois Lane in LOIS LANE #7 (1959).
(9) Of course Lana was introduced much earlier as a Delilah-figure in SUPERBOY, who was perhaps worse than Lois in terms of trying to uncover Clark Kent's secret ID. Her most charmingly devious outing appeared in a tale wherein she deliberately exposed Superboy to red kryptonite to see if he'd mutate in some way and thus reveal his ID. That time it made him unable to control his powers, so he went around wrecking everything he touched. Hmm, a teenage girl making a teenage boy do stupid stuff. Who'd believe that?
(10) Sticking with the supermythology, I come to the first appearance of Lesla-Lar in ACTION COMICS #279 (1961). This tale was the beginning of a four-part plotline in the Supergirl backup feature which was a good deal more inventive than most of the stories in the Superman lead. This Jerry Siegel creation was a good if not exceptional villainess, but her tale's main historical significance is that it's one of the first multi-issue superhero tales to appear since the 1940s-- a comics-format that Marvel would later realize to greater lasting effect.
(11) In HAWKMAN #13 (first series), Hawkgirl, one of DC's more dynamic Silver Age heroines, gets to unequivccally rescue her male partner. Queen Alvit, an elvishly-named immortal who looks like a Nordic Valkyrie, tries to force Hawkman into becoming her new husband, and Hawkgirl bitch-slaps her pretty good.
(12) In AQUAMAN #18 the title hero weds his powerful girlfriend Mera, about a year before Marvel's better-known wedding of Reed and Sue Richards. One interesting historical consequence is that this bond makes Mera more a part of Aquaman's team rather than less. Precisely the reverse happened in FANTASTIC FOUR, though admittedly Mera's pregnancy wasn't milked for as much sentiment as Sue's.
(13) In JLA #60 perennial JLA foe Queen Bee masters the Leaguers by turning them into pretty little butterflies, and they're only saved from servitude by a fluke.
(14) In QUESTION #1 Lady Shiva beats the crap out of Vic Sage and almost kills him, but then decides to rescue him and give him a new life.
(15) In ADVENTURE COMICS #304, founding member Saturn Girl uses her telepathic powers to force everyone in the Legion to vote for her, so that she becomes the second member to be the Legion's leader, and the first female character to lead a team of mixed-gender superheroes. (Maybe even any mixed-gender team prior to this 1963 tale!) This mass brainwashing sounds like the act of a supervillain, but it turns out she did it to save Earth from a dire threat. And though Lightning Lad one-ups her by sacrificing his own life against said threat, the Legion votes to keep her as their leader anyway.
(16) In WONDER WOMAN vol. 1, #28, the horribly named minor character "Eviless" wins fame by forming a cadre of Wonder Woman villainesses. This is probably the first all-female group of villains in comics, or at least the first assembled from characters who had made previous appearances.
(17) In WONDER WOMAN #31, George Perez and Chris Marrinan give readers a version of the Cheetah that can and does go the distance with the super-strong Amazon.
(18) Continuing in the catfight theme, in NEW TEEN TITANS ANNUAL #1 Starfire's grudge match with her sister Blackfire merits inclusion on sheer viciousness alone.
(19) I can't say DC ever broke that much gynocentric ground in their various sword-and-sorcery books, but I did think the villainess Dark Majistra was one bad mother-- particularly since she was the hero's mother, and tried to kill him rather imaginatively in the four-part "Magic Odyssey" serial.
(20) And last on my list, I give you LADY COP #1. Reason being that, although the art avoids showing things very graphically, it seems to depict the first time a DC female ever kneed a DC guy in the groin.