D'Artagnan's formal introduction to the Three Musketeers. On his very first day in Paris he has managed to offend and make an appointment of honor with all three of them. Athos is first up and naturally he has invited the other two to be his seconds. The ensuing scene is totally hilarious from D'Artagnan's dry 'your friendship is not founded upon contrasts.' To; 'But this is the man I am going to fight!' 'But not until One.' To D'Artagnan drawing his sword with a flourish on all three; 'And so, gentlemen, my apologies and en-garde!'
The theological discussion just might be the funniest part of the whole book. Which is saying something.
The entire scene of D'Artagnan coming across Aramis all ready to take the Jesuit robes, having written his thesis and trying to prepare his soul. From D'Artagnan's agony at the Latin and theology flying around, to his Get A Hold Of Yourself Man speech, which soon morphs into "I guess you don't need this letter, then," and Aramis' reaction ("Take away these horrid vegetables"), priceless.
Porthos' method of "arranging" duels. And later, his use of said method on Saint-Aignan, in which he casually destroys a chair.
Athos and Grimaud in a wine cellar...for two weeks. The best part was when D'Artagnan managed to get them out.
"Some wine!" said Athos, on perceiving the host.
"Some wine!" cried the stupefied host, "some wine? Why you have drunk more than a hundred pistoles' worth! I am a ruined man, lost, destroyed!"
"Bah," said Athos, "we were always dry."
The Jail Bake in Twenty Years After. After receiving it the Duke of Beaufort invites his jailer to dinner, tells him of one of his many methods to break out describing the content of the Jail Bake, and proceeds to capture and tie him up before breaking out.