URINETOWN is hilarious. THE PRODUCERS was also pretty uproarious the first time around, but it's gotten less funny with each subsequent viewing. I remember laughing myself stupid at the tender age of 13 when Jerry Lewis did DAMN YANKEES
But BOOK OF MORMON easily tops them all.
CHURCH DOOR TOUCAN GAY MARKETING PUPPIES MUSICAL THEATER STAPLES PERIOD OIL BITCHY SNARK HOLES
URINETOWN was indeed pee-in-your-pants funny. I think it's the most genuinely funny of the recent "let's-quote-other-show-styles-and-nudge-nudge-wink-wink-current-trends" type of show. The book really holds it together.
FORUM is really great, but REALLY a tough show to do properly, though many folks might disagree. I think it suffers from its stylistic requirements.
Of the more classic shows, though it's dated now, LI'L ABNER captures Al Capp's ridiculously whacko characters, and has more political satire couched in it (as did the comic strip) than anything I recall. I mean, go back and listen to "The Country's in the Very Best of Hands".
Of course, COMPANY was even CALLED "A Musical Comedy". So, that's a top choice for me, too. At least I found the original production wickedly funny.
She Loves Me is my favorite musical comedy, as it's a comedy and it's my favorite musical.
If you're asking about broader musical comedies, my answer would be Anything Goes. And if you're asking about musicals at the furthest frontiers of mayhem and ridiculousness, my answer would be Pardon, My English.
On the 20th Century - I remember the show getting belly laughs and applause in the middle of half the songs.
Guys and Dolls
Forum
A Little Night Music is elegant and saucy, and also gets laughs during several songs. Getting laughter from lyrics isn't easy or common. Most songs get their laughs from staging.