My two favorites are

Airplane
Young Frankenstein (Frankensteeeen)

A few I like:
40-Year Old Virgin
Christmas Vacation
The Princess Bride
Monty Python and the Holy Grail
Fast Times at Ridgemont High
Clerks


(http://www.imdb.com/name/nm0000698/)

I agree with you on Christmas Vacation, and Monty Python. I saw Princess Bride as a story rather than a comedy although I really like it and have watched it several times.

(In no "particular" order) . . .

A Mighty Wind
Animal House
Best In Show
It's A Gift
O, Brother Where Art Thou?
Take the Money and Run
The Big Lebowski
This is Spinal Tap
Walk Hard
Waiting for Guffman

(Plus some already mentioned =)

(Did I fail to mention "A Christmas Story"?)

RM, We must really be a couple of old coots. To find another person that would list "It's A Gift" as a favorite comedy! Love it - definitely in my top 5 comedies.

I'd also go with...

Bringing Up Baby

Bananas

Father of the Bride (both versions, but I like the newer Steve Martin version best)

Annie Hall - Many wouldn't think of it as a pure comedy. But it's one great one-liner after another.

Groundhog Day

National Lampoon's Vacation

40 year old virgin and almost embarrassed to admit it Team America

Expelled

There's Something About Mary

Take the entire Christopher Guest ensemble films mentioned by Radical Moderate, and then throw in another film far, far more crude... Borat. Yes, I have to admit, I have never laughed so hard in my life as I did during my first screening of that. I had to go for a second viewing because we were laughing so hard the first time we missed the majority of the jokes.

Certainly National Lampoon's Christmas is among them, as is Blazing Saddles and a few others of that genre.

Lots of good ones

Horrible Bosses

Moderate just mentioned this in the other movie thread but let me put Bad Santa in a tie with 40 Year Old Virgin

Billy Bob was born to play that part

Yes. (and i think Bad Santa is hilarious)
But before that, he was a Serious Actor. =)
(ever see the one where he was a scary barber c/o the coen bros?)

(We actually drove to a movie house in a mall to see that one.
I can't remember the title. It was Quail Springs Mall.
The traffic and the parking were just as bad as if it had been Christmas. =)

Smiley Face.

I would have to add "Rock Around the Clock" as the funniest "Comedy Noir" film ever
(and Sam Katzman as the pretender to the Coen Bros Throne, Producer-wise)
except it--"Rock Around the Clock"--wasn't intended as a comedy.

Unlike "It's a Gift". =)

Of course, to be fair, W.C. Fields is no Bill Haley.
And vice-versa, with or without Comets or Promoters. =)

The old Hope-Crosby On the Road movies were pretty good too.

Dumb and Dumber, 40 year old virgin, Bridesmaids, Revenge of the Nerds.

Here are a few that crack me up every time...

Blazing Saddles
Office Space
Coming to America
Caddyshack
Christmas Vacation
Best In Show

if i could pick one and only one, it'd surely be airplane. blazing saddles would probably be a close second.

best one i haven't seen mentioned yet... mash, the movie.

-M