My favorite comedy is Bringing Up Baby.
Other favorite comedies, by no means a comprehensive list and in no particular order. I'll admit upfront that I spend too much time checking out Turner Classic Movies.
My Man Godfrey - my closest rival to Bringing Up Baby
Duck Soup
Ruggles of Red Gap
The Court Jester
The Lady Eve
Ball of Fire
To Be or Not to Be (Ernst Lubitsch/Jack Benny version)
Some Like It Hot
Bedazzled (Dudley Moore and Peter Cook)
The Producers
Young Frankenstein
Monty Python and the Holy Grail
Bull Durham
Groundhog Day
I saw The In-Laws for the first time late last year. Great stuff, my only question is would it hold up to repeated viewings. I suspect it would.
I'd also be inclined to throw in something by either WC Fields or Buster Keaton, but it's hard to narrow them down to one movie.
My wife's favorite is The Producers. She would also throw in Arsenic and Old Lace and Best In Show. Both are worthy.
A couple others I would add...
1) My Favorite Wife....Cary Grant, Irene Dunne, Randolph Scott.
2) It's a Gift....WC Fields was never better
and a couple I purposely left out....
The Apartment...Jack Lemmon, Shirley McClaine, Fred MacMurray, Ray Walston, Jack Kruschen......
.....more drama than comedy, but some hilarious moments. It also deserves to be on it's own list. In it's own way it said so much about getting ahead in american business and the general view at that time in american history, and it's uncanny criticisms of the unwinding of american morals in view of the late 1960's sexual revolution which was still several years away. All the while telling one the best love stories ever put on film. In my view, one of the top three films ever made in any category. Some tough competition, but IMO, the great Wilders masterpiece.
Dr Strangelove....Peter Sellers....
....A dark comedy-satire, not something I would lump with laugh a minute comedies.
The Fortune Cookie...Jack Lemmon, Walter Mathhau.
A great film but another "dark" comedy-satire. That being said, Walter Mathhau is unforgettable in this film.