There aren't a lot of laughs in the Haunting, but a true classic comes with Luke's remark: "Hey, Doc, ya wanna buy a haunted house? I'll sell ya one, cheap!" It works all the better considering that it follows the breathing door scene.
There's also the part were the group finds the library's collection of books full of creepy demon illustrations. Luke considers using them to make Christmas cards.
Another one from Luke, as his skepticism begins to unravel:
Luke: "I haven't seen a damn thing!" (beat) "I just don't like the way it looks."
One from Markway early on when he finds the two girls lost in the hallway:
Markway: "Now, which door?"
Theo: (considers, then points) "That one."
Markway: "Wrong. I've studied the map, it's this one."
(Markway confidently walks through the door...and into a broom closet).