The lusty argonian maid

The Princess Bride is one of my favorite examples of this, especially because the "story within the story" is the main story, which is unusual.

I'm surprised no one has mentioned the fake old movie that plays in Home Alone. "I'm gonna give you till the count of 10 to get your ugly yeller no good keester off my property before I pump your guts full of lead! One... Two... Ten!" 🔫🔫🔫🔫🔫

Tales of the Black Freighter from the Watchmen comics is pretty awesome.

All My Circuits on Futurama is one of my favorites on tv. Dramatic beeping intensifies.

Ow! My balls! from Idiocracy

There's been some stuff coming out lately that makes me think this show isn't far off.

Angels With Filthy Souls

The "adult" movie Kevin watches in Home Alone. Apparently the main dude who was supposed to be in those didn't show up so they just had some janitor or tech fill in and he went full ham on that shit and made it something to remember.

There's also the McBane movie in The Simpsons that shows up in multiple episodes and if you connected them all together they actually make a coherent story line (it's just riffing off Lethal Weapon anyway).

Community has Inspector Spacetime and the Kickpuncher movie series.

30 Rock has quite a few good ones:

• M.I.L.F Island
• Bitch Hunter
• TGS
• The satirized version of NBC in the show (lots of 'biting the hand' humor)

"The Hitchhiker's Guide to the Galaxy"

by Douglas Adams is a book about a book called "The Hitchhiker's Guide to the Galaxy"

Wormhole X-Treme! from Stargate SG-1

Tool Time in Home Improvement

"Sick, Sad World" from Daria

Interdimensional Cable from Rick and Morty is outrageous. GTA's radio stations (VCPR was the best) and TV shows are often really funny. The Pißwasser beer commercial from IV always gets me.

Control: Threshold Kids

and Bitch Hunter from 30 Rock

Check out https://nestflix.fun/ for a Netflix-like way to browse popular fictional media like this

The "I'd buy that for a dollar!" guy from Robocop.

Ballfondlers from Rick and Morty

Impotent Rage from GTA V

The Silver Shroud from Fallout 4

Tropic Thunder movie previews

“Scary door” from Futurama

It’s a play on the twilight zone and it’s quite something.

“A casino where I’m always winning? This must be heaven!” “A casino where I always win… I must actually be… IN HELL!”

“No Mr. smith. You’re not in heaven or hell. You’re on an airplane!”

“Help! There’s a gremlin destroying the plane! You’ve gotta believe me!”

“Why should I believe you?! You’re Hitler!”

For those interested: The Scary Door

"The Wall" in Solar Opposites. It's arguably better than the main story.

The Adventures of Captain Proton from Star Trek: Voyager!

"Ya Heard? With Perd!" in parks and recreation.

If games count then all the radio stations in GTA

The book House of Leaves is presented as a documentary within a book within a book. Really fun read, too

Mysterious Mysteries of Strange Mystery from Invader Zim

Come to Zim!

“Invitation to Love”, the soap opera within the show “Twin Peaks”.

McBain in The Simpsons is a classic.

This video stitches the movie together somewhat.

https://youtu.be/BzjfrdcN1Z8?si=G84IdUwMkfPKDm3w

HYPNOTOAD

the Wall in Solar Opposites.

Teen Girl Squad!

The lore books in The Elder Scrolls series, hands-down.

There is an entire universe of conflicting knowledge, personal bias, and unreliable narrators that leave Tamriel's history feeling very real, and very open to interpretation. The fun of it is piecing together the truth somewhere in the middle. But I'll die on the hill that the Arcturian Heresy is absolute horseshit written by a madman, and comparable to the scribbles of a paranoid schizophrenic on an anti-vax forum. Anyone who references that volume in regards to Tiber Septim and the forming of the empire is an impressionable dweeb.

Three Dog, the radio DJ for "Galaxy News Radio" within Fallout 3, was one of the best parts of the game.

The Fallout series has lots of other media within media too, like the Grognak the Barbarian comic series or Cat's Paw magazine.

This is not exactly what you're asking for (media inside media), but it's really close in spirit (nested narratives), and I really like it: a book written in Portuguese in the XIX century, called Noite na Taverna (Night in the Tavern).

The book has an overarching story of friends telling each other stories in a tavern, over booze; with all those nested stories being about love, despair, and death (it has a strong gothic vibe).

And, as each character tells the others a story, there's always that fishy smell that the story might be actually bullshit; and other characters do raise some doubts about its in-universe veracity (like Bertram does to Solfieri). And you, as the reader, do the same - but in no moment you question the veracity of the overarching story, and you feel like you're inside the tavern alongside the drunkards.

So it's a lot like the author is toying with your suspension of disbelief - redirecting it from the overarching story to the nested stories, and as you doubt the later you get even more immersed into the former.

If I must use an example of media within media, then my choice would be "The Book" within Orwell's 1984. I think that it's a great piece because it shows Orwell's views on politics and society, while still serving narrative and worldbuilding purpose - for Winston it's a material proof of the Inner Party's bullshit, for O'Brien it's a tool of the Inner Party to sniff out dissidence. (Note: 1984 is extremely misrepresented nowadays, I'm aware, but I still like it.)