The moment that kickstarted the series: Mandy literally cheats Death himself.
Grim: Oh uh, any last words before you lose? Mandy: Just two: kiss kiss. (cue hamster attack)
Grim and Mandy replace Billy in one episode with an evil doppelganger named Bobby who attempts to completely take over Billy's life. Billy, in an ultra-rare moment of intelligence and cunning, steals Grim's scythe and banishes Bobby to another dimension.
Grim: The Grim Reaper does not like being tricked! So I decided that Jack won't be showing his face around town again. EVER!
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 whence you came!
Grim: Okay, okay! (quietly) You little monster.
From the same episode, Pandora calling Mindy a loser, which immediately earns Mandy's respect. Mindy's reaction makes it all the satisfying.
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 to steal from me. Do you? (cracks another nut with just as much force)
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.
Also points for Grim and Billy cursing Mandy with a mask that will turn her into a donkey-like beast (a literal jackass) unless she acts nice to them. For perhaps one single point in the show they actually had her under their thumb. Granted she eventually sacrificed beauty to beat, in this case, literally kick the living hell out of them, but even then they insist it was Worth It.
Mandy stands up to her own nerve (the source of her dominant personality which was stolen by Billy) 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 broken me? Humiliated me? Driven me crazy? When every single day of my life is dealing with: (cut to a montage of Billy doing random stupidities). Compared to that, you are nothing more than a pimple on the butt of the world's smallest amoeba, 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.
When the duck frames Mandy for using the intercom, PrincipalGoodvibes, of all people scares Mandy into submission. Next we see her, she just got back from her second week of detention.
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.
Mandy: "Wow... Mrs. Doolan beat Grim. She beat death."
It wasn't just staring contests. She beat him in, and I quote, "Just about everything". Some of the examples she listed were Tennis, Golf, Boxing, and Poker, the last of which she won Grim's Dog, Lucky.
In GoodBling and the Hip-Hop-O'potamus, Irwin's Granmama beats Principal Goodvibes in a "Yo Mama" throw down. Some of the stuff she said doubles to Funny Moments as well.
Granmama: Yo mama is so ugly she made onions cry! ... Yo mama is so stupid she took a ruler to bed to see how long she slept! ... Yo mama is so stupid she cheated off Billy'stest!
Though it's ruined a few moments later, Grim reminding Billy's Mom just exactly who she's messing with is rather awesome.
Grim:Foolish mortal! I don't think you know who I am. (Magics up his scythe) Let me give you a hint!
In "Scythe For Sale" Irwin gets his hands on Grim's scythe and goes mad with power, turning Grim into a slug and hypnotizing Mandy into liking him. Billy and Grim go to get the scythe back and rescue Mandy and when Irwin thwarts Grim's attempt to snap Mandy out of it, Billy comes to the rescue by taunting Irwin to zap him with the scythe and dodges every blast sent at him.
Billy gets another one a few seconds later. After Grim fails to knock the scythe out of Irwin's hands by catapulting rocks at him, he states that they need something harder than rocks. Billy, remembering a previous remark from Grim, immediately pulls out a plate of homemade sandwiches that, when launched, successfully knock the scythe away.
In "My Peeps" Grim performs magical corrective surgery on Billy's eyes multiple times causing him to see the world in several different art styles. After stopping on one that makes the world seem like a Dr. Seuss story Billy thinks Grim and Mandy are laughing at him and loses it, chasing them down with Grim's scythe.
In "Secret Snake Club", Billy joins the CIA, and proves to be quite competent, chasing down a criminal and even exposing Irwin's diabolical plot in the macrame club.
Grim's guitar playing in "Battle of the Bands," which is so epic that ground cracks and demons erupt from the bowels of hell.
In "Keeper of the Reaper", Judge Roy Spleen manages to scare Mandy into being quiet by threatening to put her in the chocolate pudding chair, one of the few beings who can get Mandy to behave.
Judge Roy Spleen: The stains WILL NEVER COME OUT OF YOUR CLOTHES!
Harold gets one in "Billy Idiot," where he not only figures out the evil plot of a ballet dancing dark sorceress quicker than Mandy did at the start, but then manages to destroy her by beating her in a ballet dance-off.
Grim, in general, is just a walking skeleton of pure awesomeness.
The way he confronts Sperg for the first time in Billy and Bully is a prime example.
Grim declaring Harold to be Billy's problem and the reason the boy is stupid in The Problem With Billy.
Billy and Mandy's Big Boogie Adventure:
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 the Hand wasn't even activated at the time! All Grim 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!
Irwin saving a hypnotized Mandy from Boogey's ship, curbstomping Boogey's crew with The Power of Loveand finally gets to kiss Mandy, which breaks the spell she's under.
Wrath of the Spider Queen:
Jeff the Spider epically Calling the Old Man Out after Billy calls Velma his girlfriend instead of his fiancee. 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 cuttingmy brake cables and I've had it!
Throughout the flashback scenes, Grim had to put up with a lot of Boogeys taunts and bullying... but its after he indirectly costs him his friendship with Velma during the Reaper election, that Grim finally snaps. He grabs the prototype scythe out of Principal Spleen's hands, cuts a huge crack in the ground, unleashing a huge pillar of green fire and demonic spirits that proceed to give Boogey a well-deserved beatdown. He then tops it all off by letting out that Evil Laugh we all know and love, thus acting truly scary for the first time in his life, and winning the Reaper election by a landslide. And it. Is. GLORIOUS.
Underfist: Halloween Bash:
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 simply eating it (the monster itself was made entirely out of candy).
Hell, Underfist was one walking CMOA what with 5 popular recurring characters becoming a badass team of heroes.
General Skarr manages to get one. The team has been defeated, Skarr, beingSkarr, has betrayed the good guys to Bun-Bun and all hope seems lost. Then, suddenly, Skarr betrays Bun-Bun, and saves the day. Turns out, if you're good at betraying others, that's still a power that can be used for good instead of evil.