One of mine would be from "German Week" when he says to Mrs. Slocombe...
Mrs. Slocombe "I remember being flat on my back... and the German Air Force was responsible !"
Mr. Lucas "The rest of the time she was flat on her back, the American Air Force was responsible !!!"

And of course, he's made countless comments on Mrs. Slocombe's figure!
Then he laughs when he realizes he's "ok".