Well there's the 1980s one so why not a 1990s? 
For me there are a few:
When Victor Meldrew picked up a dog instead of a telephone in One Foot In The Grave.
In Next of Kin when Maggie pushed a trifle into Georgia's face after she faked her birthday.
2point4 Children when they spoofed The Prisoner and when Bill and Rona got trapped by live cobras.
Goodnight Sweetheart when Phoebe came into the 1990s
I'm Alan Partridge - almost too many to mention, but the stalker episode (To Kill A Mocking Alan?), where Alan has to pretend a fan's house is his, has to be in the running. I especially liked the use of the Aerial liquid ball as drinking vessel. That same episode also popularised use of the word 'mentalist.'
Actually, Partridge must get the 2000s too. For DAN! DAN! DAN! DAN! DAN! DAN! DAN! DAN...DAN! DAN!
Quote: Ennie @ January 20 2010, 1:04 AM GMTI'm Alan Partridge - almost too many to mention
The cow.
Peter Baxendale-Thomas.
Cock Piss Partridge.
I mean, people forget that traders need access to DIX-ONS!
Again, Alan Partridge - 'I've got an udder on my leg' particularly tickles me.
One Foot in the Grave - Mrs Warboys gets back in the car after having a wee halfway through Beast in the Cage.
LoG - Geoff Tipps best man speech.
The Club from Bang, Bang It's Reeves and Mortimer (it's a mini sitcom, sort of) - Kinky John losing the plot at the end. 'I am a human insect'
Quote: Tim Walker @ January 20 2010, 10:26 AM GMTDel Boy falling through the bar.
The repeat of the episode of Del Boy falling through the bar.
Just play the f**king note!
The first one?
No not the f**king first one! The f**king first one's already f**king down! Just play the f**king note you were f**king playing earlier! I've been playing the f**king first one! We have the f**king first one!
So I'll just...
Just play the f**king note you were f**king playing there! F**king what you were just f**king doing! Play the f**king note!
The first appearance of Ace Rimmer.
What about Partridge singing ABBA, with the character played by Rebecca Front?
For me it has to be Traveling Wilburys 'End Of The Line' musical sequence for the last ever 'One Foot In The Grave', nice to see it go out with laughter, and how all those comedic moments fitted in with that episode!! Sad but Brilliant!!!
In light of the raising of the Terror Threat Level to severe, I'd like to add this from Red Dwarf:
Rimmer: Step up to Red Alert!
Kryten: Sir, are you absolutely sure? It does mean changing the bulb.
The clubbing episode of Spaced.
The Hitler moustache bit of Father Ted.
George Dent waking up on a bench in train station, the morning after the office Christmas party (Drop The Dead Donkey).
Quote: maidenpriest @ January 21 2010, 2:16 PM GMTFor me it has to be Traveling Wilburys 'End Of The Line' musical sequence for the last ever 'One Foot In The Grave', nice to see it go out with laughter, and how all those comedic moments fitted in with that episode!! Sad but Brilliant!!!
Great show, and a wonderful moment, of course, but without wishing to appear awfully pedantic, wasn't the very last episode of One Foot In The Grave actually shown in 2000?
Indeed it was. November in fact.
Quote: Gerry McDonnell @ January 20 2010, 12:01 PM GMTJust play the f**king note!
The first one?
No not the f**king first one! The f**king first one's already f**king down! Just play the f**king note you were f**king playing earlier! I've been playing the f**king first one! We have the f**king first one!
So I'll just...
Just play the f**king note you were f**king playing there! F**king what you were just f**king doing! Play the f**king note!
What is ittt?
Quote: zooo @ January 23 2010, 5:56 PM GMTWhat is ittt?
http://www.youtube.com/watch?v=jzYzVMcgWhg
Fecking great episode. 
My vote: "Tell them the circus is coming to town"