Fawlty (natch): wading through trifle, followed by: "Duck's off, I'm afraid"
Favourite Joke/Moment from a Sitcom? Page 2
Quote: Frankie Rage @ November 1, 2007, 1:30 AMBSG: Aaron: I'm not too geeky am I? / Mark: 'course not!
You don't know how accurate that is. 
The 'there's cocaine in them' scene in Father Ted where Mrs Doyle wants Ted to eat some cake is mine. I also love the small/far away bit mentioned above. There's also the bit in Coupling episode 'The girl with two breasts' where Jeff turns around to Steve and gives the thumbs up and says 'breasts’ with a grin on his face, then Steve returns the favour. Also when Jeff talks about sexually assaulting the telly during Songs of Praise. There's also many many more. 
Quote: earman2009 @ November 2, 2007, 7:20 PM(...) Also when Jeff... (...). There's also many many more.
Now that you've opened the Jeff chapter:
The moment when Jeff realises that he's still wearing the black mask and that he's accidentally robbed a pharmacy ('That explains why they were so helpful at the chemist's.').
In the same episode:
JEFF: Normally when I ingest some of Julia's jewellry...
STEVE: Jeff, "normally" has never been used in that sentence before!
or:
JEFF (arriving at the bar with the black mask on): How long do intestines take?
Oh no, now I have to watch it all again. And there goes my research paper...
°Finck.
One Foot in the Grave, Victor sitting in his chair, phone rings, reaches down outside of chair, picks up phone shaped puppy (dachsund).
Just the best.
4291?
The best comedy, is where the joke is built up in a crescendo of laughs. Fawlty Towers did this as a set formulae. Coupling, too, was a series of intricate crescendo's.
That is, without the build up of humour, the 'sitcom moment' doesn't work as well.
FAWLTY TOWERS
german man: would you stop talking about the war
basil: me, you started it
german man: we did not start it
basil: yes you did, you invaded poland
Quote: feck, arse, drink, girls, gobshite @ November 3, 2007, 9:48 PM(...) you invaded poland.
That reminds me of:
Ted and the perfectly square black spot of dirt on the window.
Just great.
°Finck
Frasier is one of my favourite sitcoms, full of brilliant moments. The one that sticks in my mind is when Frasier and Niles are reminiscing, and mention a time during which they were 'playing air violin'.
I have to say i've not seen that, but i can picture it, and it looks damn good.
Quote: Badge @ November 1, 2007, 2:15 AMFawlty (natch): wading through trifle, followed by: "Duck's off, I'm afraid"
Damn! You beat me to it. Love it when Fawlty slams the lid back on the trifle, desperately hoping it might disappear! 
Alan Partridge playing air guitar in his caravan takes some beating!
One of my all time favourites is from the Young Ones. The episode "Bambi", when they're on the train and Neil is testing Rick.
I won't type out the whole thing but it finishes with:
Rick: Hang on, don't tell me, I know this.... oh alright, what's the answer?
Neil: You just said not to tell you.
Rick: No I didn't!
Neil: You did just then. You said 'Hang on, don't tell me'.
Rick: Yes, but I only meant for a minute.
Neil: A minute from then or a minute from now?
Rick: Just SHUT UP AND TELL ME THE ANSWER!
Neil: Shut up and tell you the answer?
Not quite word perfect but that's the gist.
Also in Blackadder in one of the episodes with "Bob" the girl pretending to be a boy, there is a sequence which is a parody of an advert for a romantic compilation album. One of the song names that scrolls along the bottom was "Hot Sex Madrigal". A friend and I laughed for 10-15 minutes straight and having to pause it. We had been doing medieval music in one of our courses at university in which Madrigals were mentioned so it was extremely funny at the time. 
Quote: Mannikin Bird @ November 2, 2007, 9:20 PMOne Foot in the Grave, Victor sitting in his chair, phone rings, reaches down outside of chair, picks up phone shaped puppy (dachsund).
Definitely among the best moments.
My fave from One Foot, however, is when Victor goes into WC, runs out shouting and screaming in phone to someone about the delivery problems... camera pans over to WC where there's a palm planted in toilet bowl.
I always need to pause it.
I've mentioned my fave moments from Blackadder already - both times when Blackadder is at total loss for words and stares without moving a muscle: Series 2, when Percy and Baldrick giggle about the turnip shaped as thingy, and Series 3, which goes as follows:
Amy: He says my nosy is tiny?
Edmund: And wee, madam.
Amy: Well he must be an awful clever clogs, because you see, my nosy is
tiny, and so wee, that I sometimes think the pixies gave it to me!
Atkinson's reaction is beyond perfect.
Then there's uplifting, yet heartwrenching Moondance in Frasier... the trembling Sir Humphrey in "Skeleton in the cupboard" episode... Nativity play in Vicar of Dibley...and every time Hawkeye suffers from severe hangover.