You should definitely check out all the Mel Brooks stuff... Young Frankenstein, Blazing Saddles, Spaceballs ...etc...
Groundhog Day is a good one too. Not that old though.
Oh yeah! That's a good one too! :)
Around Christmas time, watch Christmas Vacation. It never stops being funny.
The Marx Brothers films
The Out of Towners (1970) Jack Lemmon, Sandy Dennis
It is much better than the remake (with Steve Martin and Goldie Hawn).
The Party with Peter Sellers. Hilarious !
I don't know how old this is but I love Flubber. It's my earliest memory of a film and it is just hilarious! I forget what actually happens in it though. I haven't seen it in years.
Oh right. Sorry!
Ferris Bueller's Day Off and Denis The Menace still make me laugh even though I've seen them a million times.
dumb and dumber. i like it because it's so flippin' hilarious.
Pee Wee's Big Adventure. Funny as hell.
Zoolander. So feckin funny.
Happiness. starring jon lovitz. is insanely funny and disturbing. it's an aquired taste. hilaroius though.
oh and spaceballs !!!!!!!!!!!!!!!!!!!!!!!!!
AND ... and... and....
Monty Python and the Search for the Holy Grail. funniest sh*t ever!
To enjoy the movie Happiness, you need to be able to find misery a little funny. If you cannot, you will be offended. : ) but I still recommend it.