I think one of my favourites is from series 1, when Duncan and his wee hippy Janice are about to snog on the beach and he gets a serious shy attack. She says "What's up do you think some wildcat will be stalking us out here?" then you hear a woman scream and enter Lexie stage left.
...and of course anything with the legend that is Richard Briers (Britain greatest comedy actor).
There are so many hilarious Monarch Moments that's is difficult to chose one.
One which is coming up in my mind right now is Dunc and Arch in the gay bar ;D ;D ;D Duncan trying to hide his knees and talking loud about women ;D hitting Archie on his back ;D gosh that was really a funny moment.
Oh my word, there are so many it is impossible to narrow them down. I loved Hector with the dogs. "Shall I fetch your stick for you?" and teaching Useless to fetch. Priceless stuff. Really, the writers were absolutely brilliant in the early days.