Comedies are like a pop song. Great in the moment, but slowly lose their magic as the years pass.
The only one I can really watch still is Caddyshack. Not that it has a ton of great laughs or is evenly paced, but the characters are just fun to watch.
Comedy is often based on the unexpected so certainly the first time you hear a joke, it's almost always going to be the most unprepared you are for it.
But I think humor that is based on general human nature and not just shock or truly just flavor of the moment references lasts much longer. There are certainly great comedies of my life (some mentioned in this thread) that I can still come back to and find really funny.As long as I don't over dose on it when doing so.
I still love classic Looney Tunes cartoons that I loved as a kid. In fact I appreciate them in a different manner since as a kid it was more about the explosive violence I just liked.