The Teensies all arguing over which one of them is the king. In the end, they simply pass the crown around at regular intervals, since they can't agree on who is the king.
Walking shells. Rayman can tame and ride them.
Rayman disguising as a purple bush to sneak into a Robot Pirate base at the end of the "Canopy" level is funny enough, but even better is what happens when their scanner detects him.
Pirate Scanner: It's okay everyone, it's just a big-nosed bush.
The Bad Ending of the Cave of Bad Dreams that you get from choosing the treasure over the medicine you need to save Clark surprisingly qualifies. It suddenly cuts to a fat Rayman relaxing on his own private island surrounded by his gold with the words "THE END" up in the sky. The game then promptly cuts right back to the choice you made to get the bad end. Stands out as a very silly and funny "What if" ending in what is otherwise one of the gloomiest levels in the game, and the suddenness of the game cutting to it out of nowhere during a fairly serious moment and then immediately going back to that serious moment like nothing happened makes it even funnier.
The Knaaren occasionally make threats during gameplay. Sometimes it's a legitimate threat, but other times...
Knaaren: Make him write bad checks!
Sometimes, Rayman will overhear conversations between the Knarren, which are a welcome source of Nightmare Retardant.
Knaaren: (loud coughing) Hey, does anyone have any cough syrup?!
Murfy has some good ones, particularly when he breaks the fourth wall. Probably the best, however, is...
Murfy: I was told that after Rayman 2 I'd be cast as a tormented artist who falls in love with a girl with great, big...eyes. And here I am, still playing the lowly sidekick. Sheesh!
The Teensie doctors can be pretty funny at times, particularly their, erm...methods for curing Globox.
The second doctor, Romeo, has one particularly glorious moment where he insultsGlobox by recommending a doctor that specializes in "hollow cavities". André is currently residing in Globox's brain. The way Romeo throws it out there without breaking character makes it all the more hilarious. He's also, arguably, the last of the three you'd expect to insult someone.
In the first trailer, when Rayman and Globox meet, they become friends by slapping and punching each other. Also, during a fight with a golem, Globox latches onto the side of its head with his teeth and starts slapping his eye.
During the final boss, the Magician, who actually turns out to be evil, falls over as his hat falls off. He takes a look at it, and then starts dancing, a la the original game. He gets Rayman and friends to follow suit, and just as the number is over, the Magician speeds off, and those present have a "Wait, shit" look on their faces. It Makes Sense in Context.
The ending segment, should you not unlock the Land of the Livid Dead, can be this. The heroes just trounced the big bad and land right back where they started for some well-deserved rest... but then the game continues, with the realization that what just happened was entirely unrelated to the issue that started this mess; Rayman and his pals snoring loud enough to literally wake the dead, and having to go back out into the world all over again to try and end the current dilemma for real.
This halloween-themed trailer featuring the Grannies and zombie versions of Rayman, Globox, and the Teensies.
Monster Voodoo Mamma polishing her nails right before you fight her. This combined with the music playing is hilarious.