The "Cuddle" interaction between a toddler and a cat in Pets; the cat will always react rather humorously to being squeezed.
The fact that Bigfoot is the supreme master of everything. Need somebody to do your gardening? Bigfoot! need a flower arrangement? Bigfoot. Need home repairs? Bigfoot! need a makeover? Bigfoot. Need a robot? Bigfoot!
If The Butler has started preparing a meal and the player cancels the action, he will cry.
Evil trait also counts, as it mentally turns your Sims into Murray.
To wit, they can "Take evil bath" "evil slumber", and "Make evil latte". Muhahaha! Also, they can steal candy from babies and make them cry.
Sims given the Neurotic trait have the option to "Freak Out", which gives them the Tranquil moodlet. It might depend on your sense of humor, but the way they scream is kind of funny.
Any Sim with the Insane trait. Many of the things they do are just hilarious.
One of the jobs for the firefighters in The Sims 3: Ambitions, should you have Late Night installed too, is to save the movie set from a herd of angry llamas.
On the Sports Channel we start with a woman reporting in the snow with some people fooling around in the background. After a sports report, we cut back to them... the woman now being stiff with cold and the background people replaced with snowmen. For added hilarity, the woman FALLS OVER, stiff as a plank.
Followed by one of the newsreaders crying over her apparent death while the other goofs about.
How about the twist ending in one of Terror TV's show, complete with sexy slow jam?
If a Sim gets rejected when asking for Woohoo, they get the "Walk of Shame" moodlet. They do an actual walk of shame and hang their head down low when walking with that moodlet.
How about the "Stride of Pride" walk too?
Female Sims walk a bit funny after Woohoo in the shower...
Having a singer yell at someone, then get a pop-up saying that person needs a Sing-A-Gram to cheer them up. Even funnier when the two sims are based off characters who are mean to each other in the source material.
You can friend-zone Sims at the "Romantic Interest" relationship state.
Fairy Sims can use the "Cheat the System" interaction on claw machines, making your fairy shrink down & raid the claw machine. Their maniacal laughter while they do the deed is what sells it.
In the official trailer for the pets expansion it shows a burglar slinking along, walks off screen then runs back being chased by about six rottweilers. That always puts a grin on my face.
One of the love letters you can get in Seasons.
"...Here's something that I made while thinking of you." A photograph is enclosed -Whoa!
Fighting: The Sims end up in a Big Ball of Violence like the first 2 games, though some of the violence is visible and it's hilarious. The loser gets humiliated. In the Late Night expansion it's actually possible to start a Bar Brawl. If a Sim is slapped they will show stars around their head.
The dramatic music that plays when the repo man comes to your house. He also does an evil laugh before taking your stuff.