Well, my favourite has to be the pant fight from series 1, episode 6 of Green Wing.
Just wonderful. And look where JRT puts his hands!
It's on youtube somewhere.
Your all time, numero uno sitcom moment? Page 2
I have a very soft spot for the moment in Red Dwarf when the ship's automatic self-destruct sequence has been triggered accidentally and the crew, after desperate attempts to stop it, find that it delivers a milk shake and a choc bar instead of a thermonuclear explosion...
... absolute classic of its kind.
Peep Show when Jeremy eats the dog.