Buffy as a series got better with diversity as the series went on, originally The Scooby Gang had just Buffy and willow as the two girls while the male side had Xander, Giles, Angel and later Oz. Though Cordelia also joined giving more diversity. When Angel, Cordy and Oz left and Dawn, Anya and later Faith showed up, the gender scale was reversed with Xander and Giles becoming the only dudes in a prominently female cast. Even with Spike and later Andrew joining.
It gets more female focused considering the dozens of Slayers that show up in series 9 and series 10 (comic).
Angel plays it more straight with Cordelia being the The Chick in series 1 and two, Winifred "Fred" joined in series 3 making it a Two Girls to a Team. However the sexism angle is averted considering Cordelia and Fred respectively become the two most powerful members of the gang, even it does possess/kill Cordelia and turn Fred into Illyria in the process.
This trope certainly applies to the villains Darla, Drusilla and Harmony being the main female vampires out of the dozens of male ones. It's even worse for the female major villains, with Glory and Jasmine being the only ones for Buffy and Angel respectively.
The Daily Show with Jon Stewart rarely has more than one female regular at a time, if that. Currently, the only female correspondents are regular Samantha Bee and the very irregularly recurring Kristen Schaal. The show's spotty record with women correspondents was lampshaded when Kristen Schaal took over the show and declared Jon Stewart to be the new Senior Men's Correspondent: "Feel free to talk about men's issues. But don't expect to be on the show more than every four to twelve weeks or so." Olivia Munn has appeared multiple times, which may make her the third regular female correspondent. The show has since recruited Jessica Williams, so the situation is improving slightly...
When The Daily Show changed hosts and became The Daily Show with Trevor Noah, Jessica and Desi Lydic were the two regular female correspondents, but Jessica departed in June 2016, leaving Desi the sole regular female correspondent until Dulcé Sloan was added in 2017. Michelle Wolf and Gina Yashere are recurring contributors and are not considered regular correspondents.
Daredevil: Season 2 has just one random, unnamed Yakuza lady. She's apparently the only female Mook in the entirety of Hell's Kitchen, since every other goon is a guy or masked and not obviously a woman. Though given the complexity of the stunts in the show, as well as the need for the Yakuza to at least appear Japanese, the showrunners likely had a limited pool to choose from.
Dexter: While there are other female officers, none of them seem to be as highly ranked as Debra Morgan or Maria LaGuerta, and the latter seems to be the only woman in a position of power, save the woman who was brought in to try and undermine LaGuerta's authority. The latter caved under the pressure of her personal life.
Doctor Who, the Third Doctor episodes, set on Earth in a male dominated military organisation, used mostly guys, with main cast members being the Doctor, The Brigadier, Benton, later Harry, and one female character; first Liz, who was replaced by Jo, who was then replaced by Sarah-Jane Smith.
Emergency!: Even today, firefighting is male-dominated. To their credit, episodes may feature female trainees or female doctors. Dixie, however, was the only female regular.
Game of Thrones: Lyanna Mormont is literally the only head of a major Northern House shown to be female, even during their war council in the Season 6 finale. In the books, at least four Houses are headed by women at this point (due to all the men dying in the war). Notably, in that same war council scene, "Lord Cerwyn" appears — in the books, House Cerwyn is headed by a woman at this point, because Ramsay killed her father and brother. The TV series actively gender-swapped female leaders into male ones — possibly to showcase Lyanna as "exceptional" (when in the books, female rulers are fairly commonplace). Furthermore, Lyanna is literally the only female head of a major vassal House (not one of the Great Houses like the Tyrells or Lannisters) who has appeared in the first six TV seasons that is both a recurring role and has speaking lines. She's also only the second female head of a major noble House to have speaking lines — the first was Anya Waynwood in Season 4, who only appeared in one episode. Even though Lyanna's mother Maege did appear on-screen in Season 1, she didn't have any speaking lines nor was she identified in dialogue (it was really just a cameo).
Hawaii Five-0 is a remake of classic Hawaii Five-O, which is about an elite squad of police in Hawaii. The original didn't have a female regular until the final season, and even then Lori Wilson only appeared in ten episodes. The remake Gender Flipped Kono in order to have a girl among the lead characters.
Homicide: Life on the Street began with only one woman, Detective Howard, in the main cast. That was a deliberate decision to reflect real-life homicide squads, which were dominated by men. As time passed, more women were added to the cast.
During the fourth season of House, the title character Invokes this when he's was told he could only hire two doctors, instead of three. He has four prospects, two of each gender. He kicks one of the women out, and tells the other, nicknamed "13", that he'd hire her if he had a slot. Later, his boss, Lisa Cuddy, informs him that he has to hire at least one woman, and tells him to hire 13. Cuddy starts to walk away, then realizes that she had just given him the three doctors he wanted.
Human Target began without any female characters, and planned to add a single female character in its second season. Averted by adding two women to the cast, making it a 3:2 male-to-female ratio.
The Principle is followed in It's Always Sunny in Philadelphia with their 5-person group. Lampshaded in "The Gang Solves the Gas Crisis", when the gang discuss their roles: Mac's the brains, Dennis is the looks, Frank's the muscle, Charlie's the wild card, and Dee's the useless chick.
While it's standard for Kamen Rider to have at least one girl as the secondary lead and a second among the villains, female Riders are few and far between. Even in series where the number of Riders gets into the double digits (Kamen Rider Ryuki, its Western adaptation Kamen Rider Dragon Knight, and Kamen Rider Gaim have about a dozen each), only one of them will be a woman (Femme, Siren, and Marika, respectively).
Kamen Rider Ex-Aid averts this. During the mid-season, Poppy Pipopapo/Asuna Karino, the lead female, becomes a Kamen Rider, then, Nico Saiba later gets her Rider form as a Ride Player. This is the first time two females transform into Kamen Riders in the show itself (not counting movies, specials, etc.).
Land of the Giants has a cast of seven humans, two of which are women. Betty was written out for part of the second season because actress Heather Young was pregnant, making Valerie the only female in a group of six.
In London's Burning there is only ever one female firefighter in Blue Watch at any one time. This is a reflection of reality, as only 3.1% of operational firefighters in the UK are female. In 1986, when the series began, there were less than ten women in the London Fire Brigade. The issue was explored in the pilot movie, when the station gets its first female firefighter and the men initially react with hostility.
Mission: Impossible (both the original and revival) never had more than one female regular at a time (though missions could and did have more than one female agent involved) - the original had Cinnamon in the first three seasons, then a revolving door of replacements in season four, Dana in season five, and then Casey for the final two seasons; in the revival Casey came first, and she was replaced by Shannon.
Mock the Week has two teams of three panelists (two of which are guest panelists) and one host. The three regulars are all male, and the relatively few female comedians means it is hard to get more than one female guest panelist.
Monty Python's Flying Circus featured almost no women, because most of the roles were played by the same six (male) actors anyway, regardless of gender. By their own admission, the Pythons brought in women like "Seventh Python" Carol Cleveland only when they needed a female character to actually be attractive, otherwise, they'd just get into drag. Both Python precursor series, Do Not Adjust Your Set and At Last the 1948 Show, featured five person casts consisting of four men and one woman.
The only major female Muppet is Miss Piggy, a glamourous diva. When she was first introduced, she was a minor character. The large cast of The Muppet Show is male-dominant, but this may be due to its slapstick nature (Miss Piggy, for example, rarely takes any of the slapstick, but she certainly dishes it out when provoked). Furthermore, the regular cast used to include other female characters, such as Janice and Hilda, but both became much less prominent after Hilda's puppeteer quit and Janice's puppeteer died, leaving Piggy. Janice herself rarely appeared outside of her Five-Man Band (The Electric Mayhem), of which she was The Chick. There have been a few other female Muppets, but their tenure is either short-lived; or they're one-off characters. A notable example is Annie-Sue Pig; a young ingénue and foil to Miss Piggy. Her appearances declined considerably after the 3rd season, although she did still appear from time to time. A number of the ambiguously-gendered monsters are noted in background material as being female; but there is no clear indication of this on the show. This applies to the puppeteers as well; in the first season, there were seven puppeteers, and only one (Eren Ozker) was a woman. Ozker & John Lovelady left after Season 1, so they held auditions for a new female for Season 2, with Louise Gold getting the part (although she was uncredited for the season). In Season 3, they hired another female puppeteer (Kathryn Mullen) but also hired another male (Steve Whitmire) making it 6 guys, 2 girls. Also, in relation to Miss Piggy & Janice, they were (and still are) performed by males. Yeah.
MythBusters generally has Kari Byron as the token female. This hasn't always been the case, however; Mythterns Christine and Jess often added a second female to the group, and the earlier episodes with the Build Team had Scottie Chapman as the third member after Kari and Tory. Grant only came on board after Scottie left the show, leaving the cast with Adam, Jamie, Tory, Grant, and Kari (plus Buster).
NCIS stars a set of four investigators, only one of which is female at any one time. They do have Abby, a lab tech who is one of two characters who has been in every episode (the other one being Gibbs).
In the second season of Prison Break, Sara Tancredi is the only female character on the main cast, since Robin Tunney (who played Veronica in the previous season) decided to leave the show. Gretchen and Sofia were later added to the main cast in season 3 to avert the principle.
In QI, one host asks questions of four panelists (three guests and one regular). Rarely is there more than two female guest panelists, and often none. Jo Brand finds herself as representing the Smurfette Principle on the show on a regular basis, and so have Sandi Toksvig and Sue Perkins.
Crew of the Orion: At the beginning of episode 1, the crew consists of four men and one woman, but Lieutenant Tamara Jagellovsk of the Galactic Security Service is assigned there during the events of the episode, changing the dynamic to Two Girls to a Team. Lt. Jagellovsk is not a crewmember, and leaves during episode 7, restoring the Orion cast to the "one woman in a team of five" dynamic.
Crew of the Hydra: General Lydia van Dyke is the only woman belonging to the crew of the Hydra and the only woman to participate in the high-command-plus-government-representative conferences.
Zig Zagged in Red Dwarf, which usually has an all-male cast of Holly, Kryten, Rimmer, Cat, and Lister. But Holly had a sex change for Series III, IV and V. After Holly is rendered male again, Kochanski is added to the crew, and is the only female of the Starbug for most of VII and VIII. For part of Back to Earth, the hologram Katerina takes up the female role, Kochanski being assumed dead.
The primary cast of Rescue Me features a dozen different firefighters on "62 Truck", but only Probationary Firefighter Laura Miles is a woman. The non-FDNY characters round out the gender dynamics some, but there's only ever one woman on the 62 Truck.
The BBC's Robin Hood was rather stuck. For the first four episodes, Marian was the only female character because there was no other reoccurring female characters in the legends. This was solved with the introduction of Djaq, a Sweet Polly Oliver in the Gender Flipped role of "The Saracen", who contributed her skills as a physician and scientist to the team, leading the series to a more permanent Two Girls to a Team dynamic that stayed even when Marian and Djaq were written out.
Sesame Street maintains an almost evenly split by gender human cast, but for a long period had zero female Muppets. Many female muppets were around before Sesame Street made a large push to get a popular one, which was 1993's Zoe. By now, the Smurfette Principle on the show is fully averted, even if the male muppets still outnumber the female muppets.
Silicon Valley was this in its first season, with Monica being the only female character of any significance. Somewhat justified, as technology companies in Silicon Valley are often overwhelmingly male. The second season introduces Laurie Bream and Carla the engineer to balance out the cast a bit.
In Smallville, the earliest version of the Justice League includes Clark, Arthur, Victor, Bart, Oliver, and Chloe. This group defied Five-Man Band, and even allowed Chloe to be romantically involved with someone outside the group.
Stargate SG-1 includes only a minority of female characters: Samantha Carter is the only woman on the team (although there is a recurring female doctor who eventually ends up getting Killed Off for Real). Even during production, the military is hardly the most gender equitable of places. Due to Executive Meddling, a sexy female thief gets added to the team in the final two seasons. Atlantis is a lot better at balancing out the roles.
Star Trek: For a world with supposed complete gender equality, this applies to most Trek series.
On Star Trek: The Original Series, the bridge crew had only Uhura, who was a Token Twofer and was also relegated to the position of space phone operator. For the time, it was rather progressive to have even one woman on the bridge. The full crew contained more women, but Uhura was the only female bridge crew member.
The original pilot for Star Trek: The Original Series ("The Menagerie") used a Two Girls to a Team dynamic, one of which was a female second-in-command. It's been said that NBC gave Roddenberry a Sadistic Choice: either keep the female second-in-command or keep Spock, but not both. Years later, Majel Barrett would quip that he "kept the Vulcan and married the woman, 'cause he didn't think Leonard would have it the other way around."
Star Trek: Deep Space Nine features only one Starfleet officer: Jadzia Dax. Since the space station is owned by a partnership of Bajorans and the Federation, the primary cast is Two Girls to a Team, with Kira Nerys acting as Sisko's second-in-command.
Taskmaster: Hosts Alex Horne and Greg Davies ask questions from a panel of five contestants. The only female contestant is Roisin Conaty.
The sitcom Taxi had lots of taxi drivers, but only Elaine Nardo, a single-mom/art-gallery receptionist until Season Four introduced Simka Dahblitz-Gravas, a Love Interest and Distaff Counterpart to Latka. Simka was a recurring character until finally joining the main cast permanently in Season Five, changing the ensemble to Two Girls to a Team.
Ann was the only female out of the five regular characters on The Time Tunnel.
3rd Rock from the Sun features four aliens sent to Earth to study mankind, after having learned some about Earth from its popular culture. They maintain occasional communication with the (male) "Big Giant Head", who decided that only one of the four sent to Earth needed to be "The Woman". Thanks to their lack of understanding of the culture they would be infiltrating it went to the hyper-masculine security officer, who thought breasts looked suitably intimidating when none of the choices had horns .
The early entries of the Ultraman franchise always had four or five male characters and one female character in the defense teams. Since these were made in the 60s and 70s, the ladies were typically Bridge Bunnies.
For Studio100's Kabouter Plop series. Kwebbel was the only female gnome seen in the show for the first couple of years until Smal got introduced in 2000. More female gnomes were seen in the movies.
As her father is the headmaster, Gwen Killerby is the only girl to attend Horace Hyde White School for Boys in The Zack Files.