During the Colonial Inn episode, the door next to Britt shuts, and he does a hilarious jump, complete with flailing hands. Even the other team members laughed when they saw it on video.
Tango in a tiara. Steve suckered him into putting it on, without telling him what it was or letting him see it. He wore it for over an hour, thinking it was a headlamp and occasionally trying to turn it on. When he runs into Jason and Grant, they crack up and he discovers the prank; instead of getting mad, he just says in a tone of awe, "You guys are good." When Jason tries to reprimand Steve for goofing around on an investigation, he is still giggling.
There's a lot of these. At one point Steve dropped a waterproof flashlight into the ocean while investigating an island lighthouse. It stayed on, sitting there in the water shining light. Or in the St. Augustine Lighthouse in Florida, where Brian had to walk up and down the lighthouse stairs several times.
Jason and Grant both getting their asses grabbed at the same time in different locations. Bonus points for making the more reticent Grant explain exactly what happened.
The bat incident at Rolling Hills Asylum, where Jason and Grant were badgered by a bat. When they got out, they sent Steve and Dave into claiming that there was some "insane activity" going on. After Steve and Dave, Jason and Grant sent the female investigators down. The girls caught on to them after the bat badgered them because they heard Jason and Grant laughing in the distance.
What makes it even funnier is that the team had to retrieve the camera in the basement, which is where the bat was
Any time there is a bat in the location, it makes the investigation funny, especially when the team has to try to overcome it
Brain's infamous "Dude, run!"
During one investigation, Brian had an experience where a chair moved in front of him. Jay later described the incident "[...] the chair dancing "Be Our Guest" with Brian."