Second your affection for Court Jester and To Be or Not to Be. Both well written, great acting, with clever plot twists.
Two on the list deserve higher ranking: the British dark comedy Kind Hearts and Coronets and My Man Godfrey. Both of these are worth watching again and again just for their superb acting and clever plots. In Kind Hearts, Alec Guinness convincingly plays the role of 6 different heirs to a dukedom.
I would add to the list the TV sanitized version of Idiocracy, along with A New Leaf, a small but very engaging film where Walter Matthau sets about to murder his wife for her money, as well as a number of others mentioned in this thread.
I didn't much care for "Kind Hearts and Coronets", and i'm not sure I recall "My Man Godfrey", though it sounds familiar.
Jerry Lois also did the multi-role parts in his movie "The Family Jewels", and I suppose it was inspired by "Kind Hearts and Coronets."
Walter Matthau was in a lot of terrific comedies. But my favorite was "Hopscotch," where he's a CIA agent, forced out of his job, so he decides to write and publish his memoirs.
Mark