The scene in The Last Straw where Greg runs with his butt pointed towards the soccer coach when he yells "GET YOUR BUTTS OVER HERE!".
Greg's short stint in watching soap operas in Dog Days.
Also in Dog Days, Greg recalls a past incident in which he thought the the sign "Under 48" Must Ride With Adult" at a water park meant people under the age of 48 must ride with an adult.
Greg's theory that Fregley is like a vampire and thus cannot come in uninvited leads to this gem:
Nosferatu: Can I borrow some flour?
Greg: Sure, it's back in the kitch - hey, wait a second!
So Greg's mom signs up for a exercise program but she quits after the first class. But Greg's Dad takes a picture of her before her first class and receives duplicates from the photo place. Then, Greg's dad decides to write "Before" on one photo and "After" on the other. It's one of the funniest jokes he's ever made in the series.
The Ugly Truth has some delicious cringe comedy: When at a family gathering, one of Greg's relatives blurts out the REAL story of how his uncle proposed to his fourth wife: It turns out he was actually going to break up with her and go out with her sister, but was too chicken to actually say anything. Then when he planned to do it at a baseball game, a banner saying "Marry Me" with her name on it came by, and while he obviously didn't send it, everyone was looking at him so he proposed. The Uncle tries to shrug it off but Greg tells us straight:
Greg: We're probably gonna be going to Uncle Gary's 5th wedding next year.
The various kids' comics in the Do-It-Yourself Book are all funny, but best of all is the blurb at the end of Greg's The Amazing Fart Police: "Next Week: The Fart Police Invade A Burrito Factory".
The lock-in in The Ugly Truth doesn't stop being funny, but the impromptu fake fart noises competition is a highlight.
When Greg's mom tells him he'll be grounded if he lies again, and then tells him to lie on the phone that she's not there. So Greg, in a case of deconstructing one of the classic Parentisms of all times, makes his mom go outside and then says "mom is not in the house right now" on the phone so he wouldn't be lying.
When Greg and Rodrick fight and Mom holds a "Summit" for them, which consists of making someone draw a picture of the bad thing they did.
Greg's picture, captioned: I will not call Rodrick names.
(the picture has Greg saying "Bunny rabbit!" and baby Rodrick saying "Waaaah!")
Rodrick's picture, captioned: I will not push Gregory.
(the picture has Rodrick pushing Greg off a cliff into the mouth of a hungry shark, while Greg says, "Aieeee!")
Rodrick playing (read: getting out of) Magic and Monsters:
"Greg Heffley will not be receiving his diploma due to his inability to skip."
In the sixth book. Greg just found out that there was a blizzard, but Rowley had called him to tell him there would be one the other day. However, Greg dismissed that thought, because, according to him, Rowley calls every year around this time to say that a blizzard is coming. The reason why is because, a couple years ago, Rowley's parents taped a Christmas special and the night they had taped it, a severe weather warning had been announced, so that was a permanent part of the recording. Greg used to fall for it, but he didn't after Rowley called him on summer vacation to tell him a blizzard was coming. Phew! Just read the two pages in the book, it's much easier than trying to summarize it!
The entire thing with Rowdy Riot energy drinks. In particular, the part where the kids who were forced to empty the cans sop up the puddles with their socks so they can still drink the stuff.
In the movie, when Rowley grabs onto Rodrick's leg to let Greg escape. Rodrick's response?
Rodrick: "Let go, baby hippo!"
The scene from the movie where Greg pees on Rodrick.
A somewhat easier to miss one involving how Rodrick segues into the story; he states that the house was torn down because of it being haunted. Seconds later, in his tale, he states that it burned down. In-universe Canon Discontinuity much?
From the second movie, when Rodrick replaces the bathroom door with a spare door from the basement when graffiti is put on the former. Scared that his parents will find out, he tells Greg, "What about the D-O-R-E? The door!?"
"I'm a little birdie, I'm a little birdie - ow, my knee bone!"
In the climax of the third film, Rodrick decides to serenade Heather. Which song does he sing? Baby.
The scene in the third film where Rodrick tries to get the attention of Heather Hills by pretending he's drowning when she's doing lifeguard duty. She either doesn't notice him or simply doesn't care, and an older man who isn't in particularly good shape dives in to rescue him instead. Rodrick is understandably freaked out, but it doesn't stop there. He then proceeds to perform CPR on him. Poor Rodrick. It really is funnier than it sounds though.
Another one in the third movie, when the new family dog Sweetie chews up the meatloaf, Greg and his dad get Sweetie to let it go. Rather then make a new meatloaf, Greg and Frank put it back on it's plate, and the rest of the family doesn't notice.