It's A Gift (1934) by El Bicho
Narrowing it down to one is tough. I could probably take part in a "30 Days of Comedy" challenge with all my favorites, but I'd like to draw your attention to this gem by W.C. Fields that leaves me in stiches every time. Fields plays grocery store owner Harold Bissonette (pronounced "bis-on-ay"), though like many iconic comic actors, he just plays his infamous person fans loved: "a misanthropic and hard-drinking egotist who remained a sympathetic character despite his snarling contempt for dogs, children and women," perfectly described by an unknown writer at Wikipedia.
After receiving an inheritance, Harold decides to head out to California with his family and buy an orange orchard against the wishes of his family, but the plot is just an excuse for some wonderful bits of silliness, such as a blind man trashing his store, his wife's relentless nagging, and his attempt to sleep on the porch. The universe seems to work against Harold if the way people treat him is any indication, and while very little goes right, he continues to soldier on.
It's A Gift features funny slapstick and quick-witted wordplay, the latter of which can be drowned out by your own laughter. It's one of the all-time greats.
Some Like it Hot (1959) by Amanda Salazar
When I think about great comedies this one always comes to mind. There is something so outrageously fun, quirky and brilliant that goes on in these two hours on screen. My favorite comedy has to be Billy Wilder's Some Like it Hot.
For those that have not yet seen this film, the story is about two musicians that witness a mob killing and have to hide away so they dress up as women and join a traveling female ensemble. Tony Curtis, Jack Lemmon, and Marilyn Monroe star in the film and the shenanigans that the two male leads get themselves into is hilarious. In the midst of posing as women, which Lemmon makes for the most awkward-looking female, Curtis falls for Monroe and begins another charade, posing as a rich businessman. Some of the best scenes involve the men running around, trying to change in time to keep up all facades. Curtis and Lemmon play their characters with such conviction, you can tell it takes true comics to be able to pull off these jokes.
There have been countless modern comedies that use the gag of men dressing up as women but Some Like it Hot keeps up the gag the entire film, creating one wild ride that still makes me laugh out loud every time that I watch it.
The General (1926) by Dusty Somers
When it comes to comedy, I tend to be a fan of witty wordplay, making something from Billy Wilder and I.A.L. Diamond or Ernst Lubitsch seem like a logical choice for this category. But I just can't ignore Buster Keaton -- maybe the greatest comedic director of all time.
Calling what Keaton accomplished "physical comedy" doesn't seem to do it justice. He threw himself headlong into all manner of sight gags and outrageous stunts, often to his own physical detriment. (While filming the equally great Sherlock Jr. a few years earlier, he fractured his neck while filming and didn't find out until almost a decade later.)
In The General, Keaton mounts a Civil War epic that could probably fit just as comfortably into tomorrow's action film category. The elaborate set pieces are both thrilling and delightful, as Keaton plays a Confederate Army reject who nonetheless gets a chance to fight for his country. If there's anything in cinema that makes me downright giddy, it's Keaton's expertly staged train stunts in The General.
Caddyshack (1980) by Shawn Bordo
I feel like this is a trick question. In some ways, I already answered this as part of "A Film You Watch To Feel Good" with The Naked Gun. And that meant that I didn't want to answer Airplane to this one. But then I thought - Airplane and Spinal Tap and their like are parodies that mock established genres and conventions. They may win the award for "The Film That Makes You Laugh The Most". But making me laugh isn't the question. I laugh easily. I probably laugh more at an average episode of Mystery Science Theater 3000 than I do at any other show. But comedy is more difficult. So, I tried to think of films that are really crafted well. That list is far shorter. I can include almost all of the Chaplin and Keaton silent films. There are films that have scenes that are brilliant - there's the vessel with the pestle in The Court Jester which is the most amazing four minutes of film from the Fifties. The "Who's On First" routine is also funny but the movie doesn't hold up as well. I've laughed at the comedy of films like Rushmore, Clerks, and Life Of Brian but there's one that stands out for me - Caddyshack.
What makes this the best comedy? I think it's the combination of styles of humor mixed with a sports film that seems to spoof films that have never been made. The Chevy Chase (trying out the Fletch character a few years early) and Bill Murray characters seem to have their own movie happening within the actual movie. Their scenes are these brilliant comedic pieces that are respites from the overall comedy that is happening around them. You add in Ted Knight and his polar oppostie, Rodney Dangerfield and I can't imagine a better crafted scenario. The closest is Animal House but this film took the lessons of John Belushi and decided to use that type of character tenfold with Bill Murray. I don't get tired of watching this film and can't imagine a time when I will. As the Dalia Lama says - Gunga galunga.