One that hasn't been mentionedThe Birdcage. I love that movie!!
Fled is that music: - Do I wake or sleep?

I agree with many of the other posts but I am surprised to not see any mention of this one yet:
The Jerk (1979) with Steve Martin
The funniest movies are yes, silly and intelligent, but honestly I feel The Jerk has what Holy Grail sometimes lacks and that's restraint. There is too much silliness for the sake of silliness alone in The Holy Grail, and I will admit I think the funniest thing it has to offer are Chapman's facial expressions.

City Lights (1931)
The Seven Year Itch (1955)
House (1977)
Return of The Living Dead (1985)
Naked Gun II: Smell of Fear (1991)
Delicatessen (1991)
Amelie (2001)

From top of my head Airplane!, This Is Spinal Tap, The Big Lebowski, Borat, Four Lions Strangely or not, Ive never found Holy Grail - or any other MP movie, for that matter - to be quite the tops in terms of the sheer laugh-out-loud ratio. The surreal, subversive idiocy of it all goes a mighty long way, however as does the piercing historical/social/political satire.
"facts are stupid things" - Ronald Reagan

There aren't any movies I would consider to be funnier than Holy Grail, but Airplane! & Spaceballs tie. I call those 3 movies the Holy Trinity of comedies.
WE ARE THE BORG. RESISTANCE IS FUTILE.

thinks
..there isn't one.
thinks
Nope, I was right the first time. There isn't one.
"No one has the right to an opinion founded on ignorance" -Harlan Ellison

The Producers is very funny, but this movie is the funniest of all time.

None.
1 - Monty Python and The Holy Grail
2 - Clerks
3 - Life Of Brian
4 - This Is Spinal Tap
5 - The Big Lebowski

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The only movie where i was able to laugh more:

1. Dracula: Dead and Loving it
   The only movies where i was able to laugh equally much:
2. The Jerk
3. SpaceBalls
4. Blazing Saddles
5. Stir Crazy
6. Airplane!
7. Life of Brian
8. A fish called Wanda
9. Napoleon Dynamite
10. Meaning of Life
11. Dumb & Dumber

None.

there's one, life of Brian

Funnier than this one?
None.
Saw it in the theatres on original release, and it still cracks me up today.

"and that, my liege, is how we know the Earth to be banana shaped."

Midnight in Paris,
The Hitchhiker's Guide to the Galaxy,
Love and Death,
Black Cat, White Cat

Gods must be crazy 1 and 2!!
Borat
Spaceballs
Naked Gun
Hot Shots 1 and 2
Evil Dead 3

I think this is one of the top but I would add 'Animal House' and I don't think anybody mentioned 'Airplane'.

There would be um .. ahah well there's no not really..perhapsnaw..not quite.
No this IS the funniest movie I've ever seen or expect to. Only Blazing Saddles is close enough to vie for the funniest ever. Holy Grail, when I saw it when it was new, was the only time I ever literally saw people fall down in the theater aisles rolling in laughter. It holds up over the years.
The next funnist films for me would be:
Duck Soup
Young Frankenstein
The Life of Brian
Animal House
[

In my opinion, the only movie I find to be even close to making me laugh as much as his movie is Anchorman.
Other than that, everything else falls short. (Though I do believe The Life Of Brian is brilliant as well).

I believe Holy Grail is the one with the funniest dialog and lines, and probably the best comedy overall.
Some other notable funny movies:
Airplane
Anchorman
South Park (movie)

The only ones that comes to mind that could be funnier are!
AIRPLANE
CADDYSHACK
CLERKS
HISTORY OF THE WORLD PART 1
OFFICE SPACE

AGREED!
"Five card studnothing wild. And the sky's the limit!"

None. This is #1.

"and that, my liege, is how we know the Earth to be banana shaped."

Anything filmed or animated from the Python troupe is on parI die for the fish tank in Meaning of Life
Some of my close seconds
Shaun of the Dead
Army of Darkness
Man with Two Brains
and anything with Peter Sellers
and on TV - Fawlty Towers (of course) and Most South Park episodes and now, I'm strangely enjoying Metalocalypse

What is considered funny can vary a lot from one person to another.
There are some very funny movies mentionned in this thread. Young Frankenstein is one that hasn't been mentionned yet.