My cat is named Monkey. Guarranteed laughs every time I tell people that.
See… grapes? Not funny. Rapes? Not funny. (The word or the act, obviously.) But stick a D on there and it’s hilarious to me.
Makes shopping for window treatments a hoot.
I nominate ‘Zimbabwe’.
No offense to anyone from or connected to Zimbabwe, it’s a fine country with many upstanding citizens. I just love saying the word. Zimbabwe, Zimbabwe, Zimbabwe…
Also–and this is actually related to the OP–‘banana’.
In Apples to Apples* there are what my group of friends have dubbed “instant winner cards”, that no matter what the context the receiver has to pick them, and one of the inherently funny Red Apples is “Flying Monkeys”.
*Essentially, a game where each player has 6-7 “red” “Apple” cards in their hand, each of which has a noun, proper or not. Then, players take turns drawing a “green” “Apple” card with an adjective on it and announcing its contents. Players then select which of their “Red Apples” most closely matches the Green Apple, and the player drawing the green apple will choose which of them gets the point. But the player can select anything, so sometimes a funny or nonsensical answer will get the points!
One case not mentioned in the linked Wikipedia article is an episode of “The Dick Van Dyke Show” in which Buddy Sorrel (Morey Amsterdam) can make a woman laugh by simply saying “shoehorn”. (The point probably doesn’t fit the Wiki article, since it’s really about the low threshold of humor that woman had).
We found that the same thing worked on one girl in my high school – if you said “shoehorn” to her, she broke out in hysterics.
I am a grown man who will admit to loving the Disney show Kim Possible because of three little words: “Mystical Monkey Powers.”
In Dungeons & Dragons (okay, so maybe I’m not that grown) I’ve repeatedly threatened my friend with making a wizard with a monkey familiar who has a fetish for summoning hordes of celestial monkeys.
Unfortunately, said friend has been traumatized re: monkeys due to System Shock 2, which feature genetically enhanced monkeys that throw cryokinetic balls of energy. After playing that game for 30 minutes, even an innocent “ook” can set one’s hair on end.
In conclusion, monkeys are funny, but magical monkeys are even better.
I don’t know about monkeys, but I think the funniest word is cavort. Cavort cavort cavort cavort…
And it’s been scientifically proven that the funniest number is 32.
Pshaw, right! And monkeys might fly out of my butt.
Now I have it on first hand experience that the number 69 will make at least a third of a room of grown-ups giggle.
According to John Hughes commentary on Ferris Bueller’s Day Off, the words “nine times” are quite funny when used together.
For myself, the word member makes me snicker. In health insurance, you often hear people talking about their member. As in, “I can’t find my member!” (not even with a microscope?) Cracks me up.
I have to admit - one bumper sticker that always makes my cohort snicker is “don’t make me call my flying monkeys” …
and in the right crowd the number 68 is even funnier.
OT: Apples to Apples is one of my favorite “normal games turned drinking game” games. It’s also especially funny because, after a certain number of drinks, one can’t really predict what’s going to be the best match according to the judge, who is also a bit drunk.
“Oook” and its accompanying behaviors is enough to make me and Acid Lamp giggle. There are lots of other silly in-jokes too, but they’re not monkey related.
OT: Mine is Scattergories: one of my few inventions amongst my circle of friends to catch is “Dirty Scattergories”: we were playing Scattergories at a party (which of course involved drinking,) and instead of going for points I started making up the nastiest answers I could (as long as they were funny, or if I couldn’t think of a “real” answer.) So of course when we play scattergories now, half the answers are ribald.
At the Brookfield Zoo, the baboon display is fenced enclosure with a big rock hill in it. We call it Monkey Mountain, I couldn’t even tell you what it’s really named.
Monkey Mountain Monkey Mountain Monkey Mountain
Just puts a smile on your face, doesn’t it?
Don’t you mean “…the tail of Monkeys…?” 
The funniest word I know is wombat.
OT: <—what the hell does this mean?
This is exactly true. It’s not an official rule in my house, but it’s a damn good rule of thumb. “Golfball-Sized-Hail” is another. So is “Kevin Costner”.
Offtopic.