Every episode of Adam Ruins Everything centers around the titular Adam attempting to disprove a popular notion about his topic, whether the episode is about the romanticism of proposals or the effectiveness of a border wall. Each time, Adam is portrayed as correct, even if he's obnoxious about it.
While most episodes are apolitical, when Brooklyn Nine-Nine does, then you can clearly see where the directors lie on their spectrum about social commentary such as feminism and policing.
Boston Legal was a vehicle for Alan Shore (James Spader) to sue and deliver increasingly-lengthy closing arguments against conservative strawmen. Frequently lampshaded, although it definitely laid it on thick at times. It was a little odd how all of these filthy-rich corporate lawyers (excepting the proto-Trump played by William Shatner) are so concerned with the plight of the common man. Early episodes were a lot more morally-grey.
On CSI: NY,Gary Sinise, a noted veterans' advocate in real life, seems to have influenced Mac Taylor's dialogue about veteran-related issues now and then. "Clean Sweep" is a noticeable example.
Harry's Law seems to be another David E. Kelley example, utilizing the characters of Harry and Thomas Jefferson as soapbox preachers in courtroom scenes.
Jack Webb was known for this. Most of Joe Friday's rants on Dragnet were Webb putting in his own views.
Joss Whedon touches on his existentialist(-ish) views in the Firefly episode "Objects In Space", through Jubal Early. Joss goes into much deeper detail in the episode commentary.
"Smashed" and "Wrecked" from Season 6 felt to some a lot like a great big 'just say NO to drugs' thing. Especially "Wrecked", which was written by Marti Noxon.
Season four's "Beer Bad" is not exactly pro-boozing either. It was written specifically to get reward money being offered to shows that dealt with the consequences of alcohol and drug abuse. This failed because the episode failed to deal with alcohol consumption realistically, instead having a magical potion in the beer turn drinkers into cavemen.
In "Sick And Tired," a two-part episode of The Golden Girls, Dorothy realizes that she has Chronic Fatigue Syndrome (something co-writer Susan Harris also has). Since the disease was fairly new at the time, most doctors hadn't seen it yet, some didn't know about it, and many of them tended to blame the patient when they couldn't find the answer. She spends two episodes trying to get the diagnosis, dealing with doctors who call her hysterical, simply say she's getting old, and going to a specialist in New York (the show is set in Miami) who tells her to color her hair. After she gets the proper diagnosis, she runs into the specialist from New York, and delivers an epic "Reason You Suck" to him, which sounds like something Susan Harris might have wanted to say to her Real Life doctors.
Munch goes off on an impromptu rant about legalizing marijuana that makes it very clear the writers support this. It's given a hilarious call-back a few episodes later when a narcotics officer who had been the recipient of Munch's rant needles him about it while they're investigating a marijuana shipment, and he starts rambling on about a conspiracy theory about how marijuana was outlawed due to the machinations of William Randolph Hearst so he could get rid of hemp.
Detective Russ DeSilva: No wonder you guys couldn't solve a murder. Detective John Munch: Well, I wouldn't solve yours.
When Munch and Lewis ran into difficulties while trying to buy the Waterfront due to their silent partner Bayliss's heretofore unknown criminal record, Bayliss starts an impromptu, Anvilicious about how unnecessary government regulations harm small businesses, before concluding "But what do I know, I'm just the silent partner." and storming out.
MacGyver (1985) pretty much turned into a show protesting societal wrongs after a couple of seasons. The most glaring was probably the one that opened with a warning about a graphic portrayal of a de-horned rhinoceros, then spent about half its running time explaining the poaching in Africa and ended with Richard Dean Anderson as himself narrating about what can be done about it. Very Special Episode, indeed.
Next time you watch a M*A*S*H episode — particularly one from about Season 7 or later — check the credits. Is Alan Alda listed as writer and/or director? Then prepare yourself for a whole lot of this.
Penn & Teller: Bullshit! is blatant in its Libertarian agenda from the very first episode. Teller has said that he relishes the show being totally-biased but somehow still fair.(?)
The pulp sci-fi TV series Quatermass, which Doctor Who owes a sizeable debt to, is so reactionary as to occasionally tip into self-parody. Its central premise involves alien mind control which targets hippies but doesn't affect old people. The other part of its premise is that there's a cult who believe that the aliens are transporting them to a utopia, when in fact they're just being incinerated. The result, basically, is 100 minutes of Nigel Kneale yelling at viewers to get off his lawn.
Is it a coincidence that the left-wing soapboxing quotient on Quincy, M.E. increased as Jack Klugman got more creative control? Hahaha, no.
Saturday Night Live sometimes has this happening, most likely because the host differs from week to week. Christina Aguilera hosted in the midst of her Dirrty phase, and about three-quarters of the sketches where she played a central role (either as herself or someone else) had her character lecturing the others on how she chose to express herself as a woman.
Gene Roddenberry, the creator of Star Trek, has a history of putting his atheistic ideals in his work. It's no coincidence that "God" is revealed again and again to be an insane computer. It becomes most overt in "Who Watches the Watchers?" where the re-emergence of religion among a Vulcan-like race on one planet (due to Enterprise crew members were seen beaming down and being mistaken for gods) is treated as a terrible thing, with much sermonizing on the evils of superstition in a long Patrick Stewart Speech before they successfully prove they aren't gods to the natives.
An In-Universe application of this trope occurs in the Star Trek: Voyager episode "Author, Author", in which the Doctor writes a holo-novel which is essentially a screed against the oppression of intelligent holograms, with thinly-disguised versions of the crew as the villains. However, the end of the episode implies that maybe the novel is in fact necessary.
The West Wing varied a lot over time — the writing staff was mostly Republican in later seasons, leading to things like Arnold Vinick being the better candidate in the Season 7 election.
The Newsroom has Aaron Sorkin spending three seasons lobbing bombs at cable news journalism. Each episode will also invariably include at least one character making a strong political statement that Sorkin obviously holds dear.
It was frequently and proudly a left-ish sitcom about the working class, and for the most part it succeeded by showing its opinionated lead characters were just average people who had flaws and failings of their own (cf. "White Men Can't Kiss", where DJ refuses to kiss a girl because she's black). But, regardless of whether or not you agree wholeheartedly with the message of "The Last Thursday In November", there's no denying it was twenty-two minutes of soapboxing about the treatment of Native Americans, with very little nuance.
The 2018 revival has the real-life Roseanne's right-wing values (having since gone to the other side of the political spectrum) leak into the show, to the point of outright contradicting older seasons.
Schitt's Creek portrays a small rural town that is absent of homophobia (which is becoming the norm in Real Life), which was a conscious decision of co-Creator Dan Levy. He has said he rejects homophobia and doesn't care for preachy sitcom episodes. Consequently, the show's queer characters simply exist and live their lives without outside negative pressure.
The Wire can be seen as one five-season-long Author Tract on how selfishness, ambition, and stupidity are keeping American institutions in a vicious cycle of incompetence.
Yellowstone: Season 3 has several moments where the plot stops cold so that The Fixer, Angela, can lecture Chief Rainwater about the plight of Native Americans — how they lost their land, and how nothing is being done to fix the systemic issues that plague the Rez, even today. This escalates to the point where the Season 3 finale has give two tracts about how the American government has broken every treaty it has ever signed, "including their most recent one with Iran."
"Reason You Suck" to him, which sounds like something Susan Harris might have wanted to say to her Real Life doctors.