I was on the phone with my sister when that scene aired, and she also had it on. I’d looked away, but then I heard her say,* “EEEWWW!!!, she just ate a rat!”* That was one big surprise, seeing Diana’s jaw stretch out like that.
X-Files: “She is home.”
Millennium: Frank listening to the Selfosophy tape’s advice to “picture something that made you feel upset, like a missed taxi, or a soup stain on your necktie.” We then get treated to a 2-minute-long flashback of every hell-vision, mutilated corpse and tortured loved-one Frank has seen over the past 2-odd seasons.
Buffy the Vampire Slayer, Season 5 (and originally series) finale, we’re treated to a recap consisting of almost every important character on the show introducing themselves. Recap ends with Buffy walking into an alley to find a vampire threatening someone, and the vampire demanding to know who she is.
Wish I could find a clip of this, but it evades me. 
For me, and apparently I’m in the minority here, the scene from WKRP was this:
Wes’ rant at the beginning of the pilot of Studio 60 On The Sunset Strip
And, also from Studio 60 Danny, speaking to Jordan:
The scene I remember from Seinfeld is Jerry returning to his apartment and find George lying on the floor with his pants around his ankles, and Jerry says deadpan “And you want to be my latex salesman.”
That and Elaine’s story about the time she put her handbag down and out flew her diaphram, said without interuption to Jerry and Marla the virgin.
It will take a while to recover all my TV memories (which go back to Howdy Doody), after seeing Torchwood’s 2nd season ending show–on BBCamerica last weekend:
Tosh: “You’re breaking my heart.”
Surely, that bit will eventually take its place among all the other scenes of all the other shows.
Eventually.
B-5: Who am I? I am Susan Ivonova; Commander, daughter of Ivan and Sofie Ivanov. I am the right hand of vengence and the boot that is going to kick your sorry but all the way back to earth. I am death incarnate and the last living thing you are ever going to see…
That gives me the chills every time I watch it.
Red Dwarf, Episode Backwards: The Cat’s last moments on the reverse planet.
Are there any series where the iconic moment has to do with resurrected zombies?? 
Doctor Who: “The Eleventh Hour.” The main bad guy has been captured by his jailers, who had threatened Earth until their prisoner had been returned. The newly-regenerated Doctor goes to the roof to remonstrate with the jailers:
“Here’s a question…is this world a threat to [you?]”
The alien shows a holo-montage of stock footage of events around the world: "No."
“Are the people of this world guilty of any crimes by [your laws]?”
Another montage: "No."
“Okay! Last question…is this world…protected? You’re not the only lot to come here,
there have been so many…” At this point a montage of Daleks, Cybermen, and other various monsters pop up. *“And the question you have to ask yourself is…*what happened to them.”
The holo-montage then shows all the previous incarnations of the Doctor. Right as it gets to his last form, the new Doctor steps through the holo-bubble, bursting it.
*“Hello. I’m the Doctor. Basically…*run.”
EPIC! :D:cool:
The Big Bang Theory –Knock, knock, knock Amy. Will you marry me?
Absolutely perfect setup and perfect ending.
Absolutely one of my all time favorite television scenes ever! Just the sentence alone writes its own comedic story.
Personally, when I think of Twin Peaks, the scene that sticks out is Leland/BOB murdering Maddy Ferguson. Possibly the most agonizing scene ever filmed for television.
Breaking Bad: “Ding! Ding! Ding! Ding…”
Battlestar: Galactica: Sharon (a.ka. Number Eight) delivering a bomb to a Cylon ship and coming face to face with dozens of her very naked selves.
Mad Men: “Zou bisou bisou…”
When I think of Seinfeld, I almost automatically think of “The Contest,” and when I do, I think of the first scene with George’s mother in the hospital (i.e., the female spongebath). I love watching his reaction to what’s going on behind the screen. This scene is probably the one in the series I ever laughed hardest and longest at.
For Frasier, it’s probably the one where his (gay male) station manager asks Niles in private if he has a problem with him dating his brother. Niles’s reaction as he slowly realizes what’s going on is priceless!
On*** Fawlty Towers***: “Oh, thank you, God! Thank you so bloody much!”
When I was teaching English as a foreign language, I used a version of this to disabuse my students of the idea of emigrating to America in the hope of living high off the hog.
I like the one where they’ve all been given 15 minutes to live, and Picard has to explain to Data what happens to you when you die. He actually summed up my own beliefs fairly well.
Reminds me of the early episode of The Simpsons where they give each other electroshocks at a “family therapy” session. That scene is hilarious!
I also loved the scene at the marriage retreat, where each spouse has to talk about the things their partner does that really, *really *annoy them:
FLANDERS: Sometimes Maude, God love her—well, she underlines passages in *my *Bible instead of hers!
HOMER: (Sotto voce) Hmph! Good thing you don’t keep guns in the house.
Star Trek (TOS). “The Doomsday Machine”:
SPOCK: Commodore, I do not wish to place you under arrest.
COMMODORE DECKER: You wouldn’t dare.
[Spock gestures to Security, and the guards on the bridge take a step forward.]
COMMODORE DECKER: You’re bluffing.
SPOCK: Vulcans never bluff.
COMMODORE DECKER: (Resignedly) No … I don’t suppose they do, do they?
Just went through this whole thread (great memories here) and have a few others to add.
Quantum Leap: How Sam always ended the program with his next leap, and his final words were always “Oh Boy.” (except one time they were “Oy Veh, I’m the rabbi.” and prehaps other exceptions).
MASH So, so many have already been mentioned. “Goodbye” is one (last scene). The black and white episodes as taped programs. From the wounded soldier’s point of view. And so many more…
Night Court Probably my favorite comedy show ever. Harry in a practical joke-off with another Judge had two of the best lines in history: “It’s okay, the motorcycle gang broke his fall.” and “Cleaver, you may be younger, you may be faster, you may even be smarter. But you will NEVER, EVER, be crazier… than me.”
The IT Crowd: When they went to see the musical “Gay”, and Roy somehow ended up faking being disabled.
The Big Bang Theory: When, in a flashback, Sheldon and Leonard were drawing up the initial roommate agreement. When it came to the clause “in the event either of us invents time travel, we agree to return back in time to a few seconds from now”, and they both look expectantly around the room. Sheldon: “Well, that’s a disappointment.”
Sherlock: The indoor swimming pool scene where Sherlock meets Jim Moriarty - chilling and hypnotic. The entire restaurant reunion scene, from beginning to end - by turns hilarious and heartrending.