The hijacking of the ice cream truck in "Operation: I.-S.C.R.E.A.M." It's the second chronological episode (which also doubles as the first aired episode for anyone keeping score) which really sets the stage for the show.
Everyone in Sector V gets to shine in "Operation: E.N.D." Their memories of being Kids Next Door operatives have been erased, but the things that make them heroes can't be removed.
Four and One incapacitate Father with a barbecue and a BFGthat shoots a mattress.
In Operation: Z.E.R.O., when they crashed the entire Moonbase down on Grandfather, JUST to get him into the decomissioning chamber so they could wipe his memory.
The very end of the series: At the end of "Operation: I.N.T.E.R.V.I.E.W.S", it looks like Father restored Sector V's memories of the Kids Next Door and interviewed them for information he needed to get revenge on Numbuh 1. It appears they unwittingly told him exactly what he needs to know, too old and confused to wonder why he was questioning them and not even fully understanding what they were remembering or saying. Then Abby gets a phone call...
Numbuh Five: Forget it! If "Mr. Uno" wants to act like a jerk for the rest of his life, Numbuh Five say, "Let him!" Numbuh Three:Numbuh Five, how can you be so mean?! You call Numbuh One selfish, but you won't even help him! Numbuh Five: Numbuh Three!... (beat) Numbuh Five: Attacking the Delightful Children wouldn't be easy. Numbuh Three: We don't care. Numbuh Five: We don't even have any of our 2x4 technology weapons. Numbuh Two: We'll come up with something. Numbuh Five: We may be turned into adultsourselves. Numbuh Four: No-one's a kid forever. Numbuh Five: All right, then — Kids Next Door, Battle Stations!
In Operation: S.A.F.E.T.Y., Numbuhs 2 and 4 taking down a SentinelExpy. Two ten-year-old boys taking down a giant robot with high-tech hedge-trimmers! And not a scratch on them afterwards.
Throughout "Operation: R.E.C.R.U.I.T." they spent the whole episode playing the Interesting Twins like a damn fiddle and tricking them into doing their mission for them while simultaneously foiling their plan. And pulling one Stealth Hi/Bye after another.
The climax of this fight was perhaps the best part. Earlier on in the episode, there was a flashback that showed a fight between him and Chad when he was a recruit and Chad was still in the KND. That fight and this one ended exactly the same way, except with Chad losing this time, showing the Numbuh One really had surpassed him and really was the best KND operative on the planet.
Taking out Stickybeard's entire armada with a single ship at the beginning of Operation: Z.E.R.O.
In "Operation: G.R.O.W.-U.P.", the adult Numbuh One blasts through a window of Father's mansion with his ice cream truck. What makes it even cooler is the music the truck makes foreshadowing the impact.
A rather underrated one, but throughout Operation: I.T. he kept his cool the whole time and was able to snap Numbuh 362 out of her Heroic BSOD.
In "Operation: E.N.D.", Numbuh 86 arrests Sector V and steals the Treehouse right in front of Numbuh One. She then makes a little speech about his mistake in underestimating her, where as she "doesn't make mistakes". Numbah One then states, "You just did.You made me angry!" He then proceeds to disarm her team with his mail and escape while they recover from their paper cuts.
Numbuh One eventually becomes a member of the Galactic KND.
Cree:A skunk? Hoagie:Not just any skunk! I've transformed Numbuh Six into a half-skunk, half-Kids Next Door operative, half-butt-kicking machine known as... R.O.B.O.B.R.A.D.L.E.Y.!
And the fact that just her scream was enough to send him and his troops running.
Mr. Boss: QUICK! RUN LIKE LITTLE GIRLS!
"Operation: B.E.A.C.H." Period.
Wally: SHE JUST OWES ME A QUARTER!
The dodgeball scene in "Operation: F.O.U.N.T.A.I.N."
Numbuh 5: "Give the kid a couple of dodgeballs and suddenly he's a hero."
Defeating the 4 Evil Twins from the Mirror Universe and then confronting his own Negative self: "Well, if you're my opposite, then that makes you a COWARD!"
Which he must have realized by just having confronted his fear of water caused by being unable to swim. Don't let it be said that Wally is as dumb as he seems. It's more of a wild card intelligence he has than anything.
Not to mention him beating up the entire negative sector V at once, and socking the Negative Numbuh Three in the face.
Negative Numbuh 1: Y-you can't talk back t-to Negative Numbuh 3 like that. I'm warning yo-
Numbuh 4: Well what are ya waiting for, princess!? (Camera narrows on his eyes) Let's DANCE!
"Operation: T.R.I.C.K.Y.": "Brick or treat, Captain Cavity!"
Beating up Numbuh 274, widely considered the best combat operative in KND history, in "Operation: E.N.D."
Numbuh Four: Don't ever call me a SQUIRT!
For all the cracks about his stupidity throughout the series, "Operation: I.N.T.E.R.V.I.E.W.S." reveals that he graduated with top marks from Harvard.
His main CMOA in the whole series. To elaborate, he is sent to a boarding school, which is apparently all-girls. 4 is informed that he would be transformed into a girl, and promptly escapes. He then disappears from the world for 75 years, after having his hand girl-ified, the majority of the human race turned into girls (or becoming girlier) and leads a new group of KND members known as the Boys Next Door. After becomeing a BadbuttGrandpa and meeting Numbuh 3's granddaughter, Future 4 leads the BND (having been reformed into the KND) into a battle against the girls and manages to help his younger self restore the timeline (at the cost of having his army turned into girls).
Let's face it: the kid's a freaking one kid army. He manages to take on his negative dimension teammates, the best hand-to-hand fighters in the series and a Crapsack World populated only by girls (as the last surviving male, with all his allies converted into females.) and win.
And his main weapon? Chewy Pellets. What was once a last request for candy became his ultimate weapon.
It's often overlooked, but he really shines in Operation D.O.G.H.O.U.S.E.: he is able to outsmart twice the villain by making use of Numbuh Five'sAchilles' Heel, and Genre Savvy enough to put a tracker in his backpack, as well as taking down the mooks as if they aren't even there. To combine this with a Crowning Moment of Heartwarming, he is extremely quick to get over his fear of what he thinks is a haunted doghouse to help Numbuh Five.
Oh, he's also responsible for taking out Chad during his first three villain appearances, the first one being when he's wrongly decommissioned.
"Five uninvited guests coming right up!"
Her first fight against Cree.
Cree; You know I was the best fighter in the Kids Next Door! Numbuh Five: That's right... you were the best!
Pretty much the entirety of "Operation: J.E.W.E.L.S."
Face it, any solo episode starring her is AWESOME.
Numbuh 362 at the end of "Operation: I.T." when she eats her way through a wall of broccoli (which is equivalent to kryptonite in this episode) and physically forces Father at broccoli-point to tag her as Supreme Leader again.
And the epic Gun Fu she and Numbuh 1 used fighting their way though a maze of Father clones.
In "Operation: F.O.O.D.F.I.T.E.", Grandma Stuffem goes from goofy recurring villain to downright legendary when she spends most of the episode kicking Sector V's collective asses and singing GWAR songs. Did you notice that the music in that episode was performed by RAWG? Stuffum was doing vocals for the real deal.
Sector Z is made of this trope! With how much butt they kick, it isn't too, too surprising at the huge fanbase they have!
Their Negative-Universe counterparts from "Operation: P.O.O.L." are no less badass.
Numbuh Zero is made of win. He's implied to have done many if not most of the same things the other operatives have done. Who could forget the line at the opening of Operation: Z.E.R.O.? Grandfather is counting down to incinerate Numbuh Zero, and when he reaches zero, Numbuh Zero says it for him and hits him with a decommissioning ray.
Numbuh 362 in Operation: Z.E.R.O. has one of these. The fact that she was able to hold back the changing from the Senior Citizombies for a full minute while everyone else was changed on contact was impressive. Compare it to how Numbuh Zero had the same resistance, and that was WITH the Book of KND in his back pocket. Authority Equals Asskicking my friends.
When Negative Numbuh Four comes into the positive world, he makes it to the edge of the pool, ready to conquer... and then he sees her. Cue asskicking.
Lizzie: Get out of my friend's world!
In "Operation: C.A.K.E.D.-T.WO." with the team down and the Delightful Children bragging to Numbuh One, he sees something in the sky and lets the Delightful Children know that they've forgotten one thing: his crazy girlfriend. Cue Lizzie single-handedly saving the entire team, getting Numbuh One to forgive her, AND ruining the Delightful Children's birthday by dive bombing the cake monster at the end of the episode.
Vin Moosk as a whole is pretty awesome, to the point he's the only adult besides Agent Zero that Nigel respects. At the climax of "Operation: K.N.O.T.", he does an Heroic Sacrifice that stops the Queen Tie for good even if it means he'll become a dull accountant again... until Nigel, in a mailman disguise, gives Moosk a can of spray starch to get rid of the tie; at the end of the episode, we all know He's Back!
There were a number of cool adults in the series: Lasso Lass, Dr. Sigmund Teeth and his defeating Knightbrace, Hoagie's mom defeating the Common Cold in her first appearance, etc.
"Operation: T.R.A.I.N.I.N.G.": Tommy and his new friends Lee and Sonia defeat Father while everyone else in the Arctic Sector got sent away. Note, that they were supposed to be on Level 3 of a simulation, and they beat them by using their smarts.
Props goes for Sonia. A sweet, little girl who looks like she shouldn't even be in training as an agent, beats up two of Father's henchmen just for the two threatening them to put the children in a broom closet. Her response after the asskicking?
Sonia: (smiling cutely) What? I don't like the dark.
Yes, "Operation: C.A.R.A.M.E.L." is not considered canon by fans. But it is pretty awesome that there is apparently a female-kid-Indiana JonesExpy flying around the world. Just a thought.
Why is it not considered canon?
Because for some reason people got upset that Heinrich turned out to be a magically transformed girl, and are still endlessly complaining the episode discriminates against boys. C.A.R.A.M.E.L. is indeed supposed to be canon, some people just took the episode the wrong way.
A minor one, but in Operation: W.H.I.T.E.H.O.U.S.E.: while Nigel's Bathroom Break-Out works twice on the Secret Service, the KND aren't fooled one second, because "we're the Kids Next Door. We invented that trick".