The opening narration done by Grim at the beginning of Underfist. It shows the Underfist logo panning slowly past the screen, with clips from the series shown inside as Grim narrates, then it zooms out, shimmers, and explodes. It oozes awesome:
Grim: When you've been around as long as I have, man, you have some crazy stories. Stories about annoying little monsters, stories about monsters who think they're people, stories about people who fight monsters, and stories about people who fight people! But the strangest story of all, is about a youth whose baby-mama was a mummy, and whose granddaddy was Dracula himself! A kid who was half mummy, half vampire, half nerd! This is his story, man, and the story of that fateful Halloween night! This is the story of Underfist!
Jeff and Fred's musical number, during which their choreography incidentally happens to beat up a large number of trick-or-treater eaters.
Billy defeating a giant Halloween monster by eating it.
Mandy in "Pandora's Lunch Box" showing exactly why she has the Grim Reaper under her thumb.
Mandy: Grim, you're gonna tell me how to stop her, and you're gonna do it now.
Grim: And why should I do that?
Mandy: * angrily grabbing Grim by the neck of his cloak* You're gonna tell it! Or so help me, (amplified with rage) I will pull your stupid cloak through your eyes and use it as reins to ride your bony butt back to the darkest pits of the Underworld from when you came!!
Also, Mandy's treatment of Skarr...
Mandy: * sitting on Skarr's own armchair, clapping sarcastically* Bravo, Mr. Skarr, bravo. I've been watching you.
General Skarr: * tries to blast Mandy with the scythe*
Mandy: Oh, really, now. Put that thing down before you poke your other eye out.
General Skarr: ....
Mandy: Since you're new here, I'll be nice and explain the rules. * takes a nut from the bowl* I am the one who gets annoyed by Billy, I am the one who bosses Grim around and, most importantly, I am the one who plays with the scythe * crashes the nut in a nutcracker meaningfully* . So you see, when you steal from Grim, you steal from me. And you don't want steal from me. Do you?
General Skarr: No! I... Wait...! This is mine! I earned it! You can't just... What are you?!
Mandy: *sinisterly* I'm just a pretty little girl.
And then, in Big Boogie Adventure. Grim gets his when the trio are trying to take Horror's Hand, which shows anyone it encounters it's worst fear. Billy and Irwin try to take it, only to be encountered and crippled by their worst fears (a spider clown mailman and grizzly bear comedy show patron). Mandy chides that they're not strong enough, but runs into her worst fear (growing up to be kind, cheerful, and married to Irwin) and is too petrified to do anything. While the three get pulverized by grizzly bears and mailmen in the background (even leading to a point where Billy weeps that it's not fun anymore..... AND FOR ONCE MANDY AGREES WITH HIM!), Grim calmly walks up to the hand and turns it off. He lives his worst fears every day (being forced into friendship by Billy and Mandy), so the Hand had no effect on him.
"I told you, you should have let me go first!"
Actually, the not so fun anymore comment came when the Boogie Man and his pirate crew managed to swipe the hand and then "seemingly" kill Grim through the hand. It's also ANOTHER awesome moment for Grim, though, because he gets another one-up on Boogie by making him face his worst fear: not being scary. And all he did was flip the Off Switch on Horror's Hand just as he picked it up.
Grim: (laughing) I totally blew me self up on purpose! You couldn't scare a puzzle to pieces!
Jeff the Spider epically Calling the Old Man Out after Billy calls Velma his girlfriend instead of his fiancee in Wrath of the Spider Queen. Okay, granted he was possessed by the Alien Cow God of Aggression , but still.
Jeff: Get away from me! I hate you dad! I hate you! I only wanted to be a good son and you never came to any of my birthday parties and you never let me see Mom, and you took all my college money to spend on jawbreakers and you kept cutting my brake cables and I've had it!
Grim acts truly scary for the first time in his life.
Mandy stands up to her own nerve without it.
Mandy gets another one. Grim has a dream that manifests a ghostly duck. The duck, (who cannot be seen by anybody but its victim), follows random people around making farting noises just to annoy them, until they snap and end up either as social pariahs, or in jail. Finally, it gets to Mandy, who not only stoically and sarcastically rebuffs every one of the duck's attempts, but then feeds it a MAGNIFICENT "The Reason You Suck" Speech:
Mandy: "You think you've won? That you've humiliated me? Driven me insane? When every single day of my life is dealing with: (we see a montage of Billy doing random stupidities). "Compared to that, you are nothing more than a pimple on the butt of the tiniest amoeba in the universe, and you have no power over me." The duck's Villainous BSOD is so great that it erases itself from existence. With an actual fart no less.
General Skarr manages to get one in Underfist. The team has been defeated, Skarr, beingSkarr, has betrayed the good guys to the Big Bad The Easter Bunny, and all hope seems lost. Then, suddenly, Skarr betrays the Big Bad, and saves the day. Turns out, if you're only good at betraying others, that's still a power that can be used for good instead of evil.
The end of Who Killed Who, when it's shown that Mrs. Doolan, The Rival to Grim (and voiced by the equally awesome Betty White.), won the fight against Grim when he came for her, and was rewarded with immortality.