Tomoya convincing Fuko that he had turned into a girl.
You are missing out all the juicy details. Fuko goes into her starfishland in her mind again and while she was spacing out, Tomoya grabs nearby Ryou and tell her to stand in front of Fuko and pretend to be him while he watches from afar. When she awakes, she's clearly shocked and asks Ryou if she's really Tomoya. After she replied, Fuko fears if she might turn into a guy. Then Tomoya shows up, laughing his ass off, Fuko's response? "There's two Tomoyas!" THEN Kyou shows up. I think Hilarity Ensues sounds like a massive understatement.
Tomoya: If she asks how that's possible, just say "It falls off sometimes."
Caption: Mastery: Ability to switch speaking to a different person.
Tomoya placing male underwear on Sunohara's head and getting him beat up for looking like a pervert.
The thing is, Tomoya wasn't expecting that rugby player to show up seconds after Sunohara left the room.
In episode 11, Kotomi swinging a toy magic wand, apparently trying to summon the Cthulhu.
Tomoya imagining Kyou holding the same wand and doing a textbook transformation sequence. What seals the deal is his hilariously genuine laughter after the Imagine Spot, and Kyou's characteristic reaction to it.
"I love fish!"
"How 'bout I punch you till you cry?"
Ryou mistaking Nagisa's attempt to recruit her as a lesbian confession, and Kyou's reaction when she finds out.
Random characters showing up in the School Festival just to kick Sunohara's ass back to Tomoyo. He gets kicked a total of 64 times.
How about the time Tomoya was the one who made the combo assist? He even gets a "Chain Combo" award by doing so.
Kyou and Tomoya getting locked in a storage closet.
Don't forgot that she thought Tomoya was going to do things to her.
Not only was she expecting him to do things to her, she was almost ready to accept it and seemed to be reasoning with herself as to whether or not she should let him.
Tomoya is given a charm that will make the first girl he talks to fall in love with him. He runs outside and he meets... Fuko. He hushes her and tells her that a bear is on the loose, and she must act like a zombie to fool it.
The "Dango-Starfish war", especially when Fuko plagiarizes the "Big Dango Family" song.
Tomoyo cooks for Tomoya to make amends for geting him suspended. Kyou, Ryou and Kotomi show up with their food which leads to a tense situation with Tomoya needing to choose which food he'll eat first. Then Fuko comes in...
Tomoya has a tough time when he tries to find friends for Kotomi. The scene with a very reluctant Kyou and the one in which Ryou shows off her fortune telling are especially hilarious.
Clannad holds the honor of making this troper laugh so hard that his throat hurt. Some highlights would be Nagisa's family's attempts to come up with a better name for Tomoya (culminating in "Okazaki Tomoya Ethanol"), the "eternal" battle between Starfish and the Great Dango Family, and mastering the art of shooting juice up a zoned out girl's nose. "Tomoya became the Fuuko Master!"
This Troper was laughing for around a full minute after seeing Sunohara approach Tomoyo in ~After Story~ Episode 2, where he's finding a fake girlfriend to convince Mei that he's committed to someone important to him. He approaches Tomoyo, who is sitting on a bench and is talking to her classmates when Sunohara sits beside Tomoyo and brushes against her shoulder. She then stops talking for a second, brushes dust from said shoulder, and proceeds to talk to her friends as though Sunohara wasn't there. Hilarity Ensues.
Sunohara: Don't ignore me like that!
The "father and son" conversation between Tomoya and Akio. Hilarity Ensues.
Funny Background Event: Sanae calmly taking the plates off the table while Akio is rolling around on top of it.
Anytime Sunohara annoys Tomoyo especially during the baseball game.
From the novel: When Kotomi plays violin for the first time we get the following.
Tomoya: THE SECOND STRIKE IS COMING! ALL CREW BRACE FOR IMPACT!
The chorus club is in chaos. The audience couldn't do anything but hold their heads and cover their ears tightly.
The K Os were the icing on the cake, especially since Akio made a really big deal out of it.
On the way to school one morning Tomoya is almost put into traction when Kyou barely misses him on her bike. Tomoya decides to confront her about this at school, cue the Clothesline from Hell by Kyou, since students at Clannad School are not allowed to ride bikes to school. Tomoya was cut off while saying -Bi- so he needs to cover it up. How does he do it?
The scene where Tomoya pulls yet another troll on Sunohara, by waking him up after the classroom has been vacated and getting him to believe the world ended while he slept, and that he is now the last person alive. As usual, it's a comedy routine in itself. Watch it here.
The whole "date" scene with Sunohara and Sanae, disguised (quite convincingly) as a 17-year-old girl, is hilarious.
Mei's annoyed reaction when Youhei suggests going to get a pork cutlet bowl is priceless.
And the pinnacle, when Sunohara found out that Sanae is married... to Akio. Well, it's understandable that Akio gets mad, but because it's Sunohara and Akio, the delivery of both shock and anger just ends up hilarious.
In an ~After Story~ OVA, Akio suggests that Nagisa impress her classmates by calling her teacher "Ultra-mom" so people will think she's related to Ultraman. This actually works, at least in a dream sequence.
Poor Nagisa getting conked on the head from Tomoya and Youhei's prank in the same episode. You'll probably feel guilty laughing at that.