I can't say as a fan that I have a favourite episode of M*A*S*H as indicated in my profile. I do have episodes that I won't watch as mentioned there because I just don't like them for whatever the reason. But I do have favourite scenes or lines.
My all time favourite was from Dear Peggy in season four, when the Koreans learning English said to Frank Burns in perfect unison "You tell em ferret face!". I have a podcast and this line forms part of the intro to it. I do have other favourites;
*Give him a high colonic and send him on a ten mile hike! (General Barker in Cheif Surgeon Who? in Season 1)
*I'm only paranoid because everyone's against me (Frank Burns in Germ Warfare in Season 1)
*Just send me some hands! Anybody! Jack the Ripper - anyone who's good with a knife! (Hawkeye in Carry on Hawkeye in Season 2)
*My kingdom for an intelligent octopus! (Hawkeye in Carry on Hawkeye in Season 2)
*Hi Frank. What's new up on the mount? (Sidney Freedman in Deal Me Out in Season 2)
*Ladies and gentlemen, take my advice. Pull down your pants and slide on the ice (Sidney Freedman in O.R in Season 2 - reprised in Goodbye, Farewell and Amen)
*The world's smallest violin and it's playing just for you. (Margaret Houlihan in Your Hit Parade in Season 6)
*Well that seems fair. He had the congressman's ear, and she got everything else! (Margaret Houlihan in Are you now Margaret in Season 8)
*The doorman was a druid! (Father Mulcahy in Mr and Mrs Who in Season 8)
*Pierce, that is a childish, totally immature and petulant suggestion.....When do we nail the swine? (Charles Winchester in April Fools in Season 8)