Katie making Micah deliver a recorded apology after he disrespects her wishes one too many times. His first attempt is so insincere, she makes him do it again.
Hunter's reaction of Ali coming inside his room after Possessed!Kristi apparently teleports away. It just seems funny seeing a baby giving "The hell is wrong with you?" look.
The pool cleaner creeping up the side of the pool is pretty silly to watch, then comes Dan's comment: "Maybe it's trying to be proactive and clean the patio, too."
"PUSSY HUNT", especially Ali's hysterical laughter throughout the whole scene. (Although it becomes less funny later on when you realise the "HUNT" part was being spelled out by the demon... it just didn't get the chance to finish spelling "HUNTER".)
Ali:(after "H" is the next letter) Pussehhhh!
Odds are Dennis' neck and back getting snapped, commando-style is either horrible or downright funny.
Every time Randy is on screen especially when the demon is beating down the bathroom door.
From the fourth film: "He's trying to cop a feel!"
Ben's web chat with Alex cutting off, and, as she leans closer to try to hear him better, she gets pulled out of bed by...! Ben, who was chatting with her on his phone just to sneak into her room and prank her.
Ben is, by far, one of the most hilarious characters in the movie series run.
Ben:By the way...you fart in your sleep.
When we see "Toby" (or at least, his avatar) in PA 4 on the Kinect, it just looks silly with the demon in boxing gear.
In 4, when Alex almost gets hit by a car as she's stepping off the sidewalk.
Alex: Fucking Prius!
Early on in 3, Dennis and the girls decide to prank Randy by pretending that there's a leak in the ceiling, and getting him to hold a pot in place below the apparent leak with a broom handle... and then they leave him there to go and play outside. When they return, Dennis pretends to throw a spider at him, causing him to jolt in fright and send the water-filled pot crashing onto his head.