What about that bit in OFITG when Victor is 'buying' a computer and all the bits from a salesman who cut him up on the ring road ? 
I can never remember which episode it's in.
Funniest sitcom moment ever Page 8
Quote: Christian Cawley @ August 20 2009, 1:28 PM BSTBlackadder the Third - the explanation of a rubber button. I mean a rotten borough.
I looooooove that bit.
'I'm a zombie!' from I'm Alan Partridge.
Couple that I like:
Hancock's Half Hour (Lord Byron Lived Here). When Hancock and Sid are going round the living room reading the supposed poems from Lord Byron under the wallpaper (see signature)
Blackadder Goes Forth. Baldrick's cappuccino for Captain Darling.
OFAH - When Rodney's making an arts film and they're writing the Rhino movie around the table.
Nearest and Dearest - When Nellie diagnoses Eli's illness as him being pregnant.
I know this one isn't a sitcom, but one of the finest visual comedy gags I've ever seen. A Buster Keaton film (the name of which I forget). Buster's working on a car and he's nailing something or other to something else. He finishes, jumps in the driving seat, starts the car and the whole underside of the car, the wheels, lights, engine roar off into the distance leaving Buster sat in the seat which he has obviously nailed to a wall. It's beautiful to watch.
There is so many. One I like though is from OFAH (Dates) I think, where the two police officers collar Del boy in the Nags head and Del tells the female officer "GET EM OFF!' The look on Mike's face is priceless.
My favourite one is "Drink up Trig, we're leaving". Classic.