There are very few films or plays that can send you (or an audience) into sustained hysterics. Those are what I cherish -- the ones that build, until you are helpless. But there are only a few in my book. I would love to hear about others:
The dinner scene in "La Cage Aux Folles" is one of those, in part because of the subtitles -- you can laugh without fear of missing much.
Most of the first compilation Monty Python movie. None of the rest ever did much for me, though lots of people swear by them.
Much of Michael Frayn's play/farce "Noises Off" -- but only if done really well.
There is at least one scene in the film "Twentieth Century" that takes off into the stratosphere -- John Barrymore describing the Passion Play at Oberammagau, complete with camel impressions and a hysterical Carole Lombard.

There is one scene in a Marx Brothers film -- not one of the famous ones -- where Groucho is seducing a woman in her stateroom -- which is complete insanity for about one minute. It just goes on and on getting loonier. I saw it with an audience once and I can remember them losing it, but only in that scene. Cannot remember the film.

"Bringing Up Baby" is maybe the most sustained film for humour-just-below-the-hysterical. It just never stops bubbling along.
The second Wallace and Gromit cartoon ("The Wrong Trousers", the one with Feathers McGaw) is the funniest thing I have seen in recent years.
yours, Peter T.