Who the primary antagonist of the Scooby-Doo movie turns out to be. (It's Scrappy Doo.)
Really, every scene with the antagonist, which make him become so over-the-top obnoxious that it loops back around and becomes hilarious.
Shaggy and Scooby taking some time to have a contest as to who can make the grossest bodily function noise, especially when Daphne walking in on Shaggy disco dancing to his own farts! So juvenile... but so hilarious!
All of the body-swapping scenes. The sheer absurdity of everyone's voices coming out of one-another's bodies and their cartoonishly over-the-top body language is hysterical.
Shaggy telling Daphne why they "Don't do castles". Bonus for Scooby acting everything out behind him, and glancing over his shoulder after Shaggy mentions being followed.
Shaggy: Because castles have paintings with eyes that watch you, suits of armor you think is a statue, and there's a guy that follows you every time you turn around! Daphne: How many times has that actually happened? Shaggy:Twelve. We're not gonna do it.
The short "instructional video" Fred and Velma find on how college kids should act, which depicts a hilariously hokey scenario about What NOT to do after having someone bump into you and spill your drink:
Nice Guy: Oh! Sorry, bro!
Angry Guy (as he breaks the glass with one hand): I will CRUSH YOUR BONES INTO DUST!
Right before Shag and Scoob find out Fred and everyone around him are possessed, hes uncharacteristically talking to them in party lingo.
Shaggy: Like, what happened last night?
Possessed!Fred: Man, we got beats like it was the lizniz on earth, ya know what I'm sayin', G?
Shaggy: (nods, beat) No.
"It's time to summon the big muchachos!"
After Scrappy Rex says he needs Scooby's soul to complete his transformation, Scooby retorts, "Me? Don't you mean Melvin Doo?!"
The DVD Commentary has Sarah Michelle Gellar spend the entire scenario with the Luna Ghost pointing out which of the backsides on screen are actually hers and which are the stunt double's.
Linda Cardellini introduces herself by her name - Sarah Michelle Gellar corrects her to "Linda Edna Cardellini" and Linda goes "I am Linda Edna Cardellini" with exactly the same amount of stress on the middle name that Sarah did.
This exchange between Shaggy and Scooby when Scooby gets mad over Shaggy's attraction to Mary Jane is so shamelessly puerile it's hysterical.
Fred and Velma dance with the possessed people to blend in. But then Fred gets so caught up at the moment, he forgets to stop dancing at the end...until he sees the Oh, Crap! look on Velma's face and realizes all the possessed people are pointing at him!
Fred gets a bit defensive when Velma accuses him of shallow tastes.
Velma: All you care about are swimsuit models.
Fred: Hey. I'm a man of substance. Dorky chicks like you turn me on, too.
The teaser trailer or Spoof Trailer where it is revealed that Scooby-Doo's shadow resembles Batman.