Raimundo combining the Sword of the Storm and the Eye of Dashi in "Shen Yi Bu", complete with Pre-Asskicking One-Liner. It's such an awesome moment that his own subconscious recognizes it as such forty-two episodes later.
What these three showdowns have in common is Raimundo's attitude during them. Each showdown follows a mistake or failing on Raimundo's part, and as such, they've activated the "brilliant" half of his BrilliantButLazy-ness. Throughout all three showdowns, typically brash, cocky Raimundo becomes calm and focused on his goal, and he caps off each Badass performance with an effortless and stylish win. The fights showcase why he deserves to be a Xiaolin Dragon.
In the episode In the Flesh, Heylin Raimundo successfully takes down Clay and Kimiko fighting him at the same time. When Omi tries to intervene with a Badass Boast, he basically just says "Nope." and escapes laughing into the distance.
Raimundo's Curb-Stomp Battle against Omi in "The Return of Master Monk Guan". When Rai goes all out, he goes all out.
Followed by The Reveal: He and Master Monk Guan effectively outgambitted Hannibal Roy Bean with a real secret elaborate plan.
In the final showdown of the series, Raimundo takes out all three of the show's major antagonists (as well as Heylin!Guan) by himself and is later promoted to Xiaolin leader.
Before that, we have all four monks plus Good!Chase trapped by every enemy. Rai promptly volunteers to drink the bowl of 1500-year-oldLao Mang Long Soup so that the others can escape, even reasoning his case when Kimiko instantly steals it because they don't want him turning evil. He would have gone through with it, too, if Good!Chase hadn't taken it, instead.
Raimundo: I am a wu dai warrior. And I am the best!.
Unlocking his wind element to it's fullest extent in the Showdown Trio with Jack and Pandabubba, pulling off a seemingly impossible come-from-behind victory that even Omi admitted was awesome.
Even before that, his Genre Savviness made him rightfully suspicious of Jack's sudden offer to team up. However, Omi and the others still looked down on him because he wasn't an apprentice and ignored his opinion in favor of the villain they've been fighting on a regular basis. Not surprisingly, the apprentices get captured with no way to prevent Pandabubba's minions from raiding the temple. Raimundo successfully pulled off a feint and orchestrated a breakout all by himself. Making a comeback win in the showdown was just icing on the cake.
Kimiko gets hers in the episode "Tangled Web", when after losing a Shen Gong Wu to Jack and having problems to control another named the Tangle Web Comb, she manages to make a comeback, fully focus on the task and use said comb to win her fight against Jack and get the SGW they were fighting for and get the one he stole at the beginnig back. For double awesome, she does this while wearing a kimono.
Kimiko: I never met an accessory I couldn't HANDLE!
Then there was that time she used the Mind Reader Conch in conjuntion with some other Wu to get the inside scoop on the groups enemies.
Ying-Yang Yo-Yo induced Heylin!Kimiko is pretty scary, and not to be underestimated in a fight ether.
When Kimiko blasts Sibini with the Eye of Dashi, combining it's power with her own fire-power in the episode "The Evil Within".
Kimiko: Just because I made a mistake doesn't mean I can't fix it. EYE OF DASHI, FIRE!
Although Chase and Hannibal thought she was Jack at the time, for Disguised!Kimiko to win a Xiaolin Showdown Trio against CHASE YOUNG and HANNIBAL ROY BEAN by tricking them into fighting against each other, which is what caused them both to lose the showdown, clearly pegs this as a CMOA for Kimiko and a very strong argument for Brains Over Brawn.
Although, her disguise almost didn't work because It is highly hinted at the end of the showdown that Chase figured out it was Kimiko who fought and not Jack
One that comes to mind is Clay defeating an Enemy Mime that was copying his every movement. So Clay punched himself in the face, causing the mime copying him to knock himself out, while all Clay got was a black eye. Dojo put it best:
Dojo: Way to take one for the team there.
Clay assembling stone armor around himself in "Treasure of a Blind Swordsman" and charging the cactus-monsters. It's not a particularly special moment, but it sure looks Badass.
Much of "Sizing Up Omi" was about how unstoppable the Cyclops is; which just makes Clay opening their showdown by jump-kicking him through sevenHUGE stone pillars all the more impressive. It doesn't end up as well as he might have hoped, but it was undeniably awesome.
Swinging around a Humongous Mecha by the end of the Tangle Web Comb as if it was nothing and sending it flying through the ceiling back at Jack Spicer in "The Crystal Glasses".
Blowing up hordes of Jack-bots by the dozens? A normal day's work for the Xiaolin Warriors. Glaring at one until it fearfully hits its own self-destruct switch? Preposterously awesome. Doubles as a Crowning Moment of Funny.
Dojo defeating the Sapphire Dragon in "Night of The Sapphire Dragon" by downing two bottles of hot sauce and breathing a gigantic stream of FIRE certainly qualifies.
Dojo turning into his giant form, coiling around the monks protectively, and breathing fire at Chase freakin' Young and an army of Elite Mooks towards the end of "Saving Omi." It doesn't come to anything, but for Dojo, it's seriously rising to the occasion.
In 'My Homey Omi' Omi and Jermaine are about to be blasted by Jack's giant robot when Dojo, giant-sized, comes out of the ground and headbutts it. That's right, Dojo became one of the Big Damn Heroes!
Dojo when he reveals his Superpowered Evil Side, as revealed in the seventh episode of season two. He takes several levels in badass, though too bad it's accompanied by several levels in jerkass too. Best shown when facing off with Jack's latest mechanical invention — even Wuya was freaking out.
Maybe it's because of his characterization, but when Jack isn't being the Butt Monkey, these moments tend to stand out. How about when he trapped Chase Young in the Sphere of Yun and took over his fortress?
When he used what everyone agreed to be the most useless Shen Gong Wu (the Changing Chopsticks) to essentially pwn the good guys and steal all of their Shen Gong Wu. The best part is the monks didn't know what hit them until it was too late.
Defeating Katnappe with the Reversing Mirror right after he had been woken up by the commotion. Despite being one of the most relatively normal characters on the show, he sure has some mad reflexes.
How about the snowboarding showdown? Turns out that Spicer isn't quite so ineffectual in challenges where snow-sports are involved.
I'm surprised nobody mentioned the part where he punched Robo!Jack square in the face, knocking him to the ground, and then ran over him in "The Apprentice". I don't know about you guys, but I found that part really surprising, seeing that this is Jack we're talking about.
Not to mention his prior face-off with Robo!Jack, in which Jack beheaded his robot double using the Shard of Lightning. Granted, it's a short-lived victory, but it's unexpectedly badass for Jack.
And of course there's Days Past. Heylin-mode Raimundo has the Xiaolin monks cornered and mockingly offers to let them join the Heylin. When they refuse, he taunts, "Face it. I'm the only one that can save you!" A calm voice abruptly replies, "Wrong." Jack then flies in with a new aircraft, completely destroys the rock-gollums surrounding the monks, saves them, then flies off, all within nine seconds. The reasons behind him taking a level in badass? He doesn't want Wuya taking over the word, because that's his job.
Even Good!Jack gets one when he instantly traps the Chi-Creature in the Sphere of Yun while everyone else was paralyzed with fear.
The beginning of 'In The Flesh', where Jack builds his Shen Gong Wu Detectobot, showing that he no longer has to take Wuya's crap.
Jack: Face it, you old witch. You're obsolete.
A subtle one that we don't get to see: In the first appearance of Hannibal Roy Bean, he delivers a vicious Hannibal Lecture, peppered with the assurance that Jack could be a great villain, if only he had Hannibal to mentor him. Jack doesn't fall for it. To put it in perspective, Jack gets tricked on an almost daily basis, while Hannibal got Chase freaking Young to do what he wanted. The insecure Jack being able to walk away from temptation like that is a testament to his Hidden Depths.
The freakin' theme song.
There's also Wuya's return to power in the season one finale; from the moment the Sun's first blotted out and the forest around her's drained of life, up to her fiery incarnation and unmasking, the broodingly apocalyptic music is perfect. And though it's no surprise now, Wuya's first appearance in a body also counts as something of a Samus Is a Girl moment: we knew that Wuya was a female ghost/witch/hag, but had no clue that she'd be the Xiaolin Showdown poster girl for Evil Is Sexy.
Master Monk Guan earns his in his showdown against Chase Young. Especially when he used an awesome Ki Attack to finish off the showdown, sending him flying through the spiral of mirrors, breaking each one on the way until he reaches the middle.
Guan: I see you now for what you are, and now what you claim to be... REPEL THE BEAST!!
While he lost in the end, Master Fung managed to last several minutes going up against a full power Chase Young, while in a full body cast and motorized wheelchair, while using only his finger and the changing chopsticks held in his mouth.
For the show as a whole: the fight between Omi and Hannibal in "Wu Got The Power," in which Omi and Hannibal are converted into beings of pure energy and bend the forces of nature around them for their fight. They clash as enormous cloud-beings while dramatic lightning strikes and the elements tear at the Earth. Even the animation becomes more detailed to match the ramped-up level of destruction.
When Omi travels back in time to witness Dashi, Guan and Chase fighting Wuya's stone golem army. All Omi can do is watch in amazement as the episode ends.
In one moment, Chase rips out the stone heart from one of the generals.
Master Fung fighting off Chase Young's jungle cats with a spinning move. Granted, it tires him out, but it sends the cats packing.
The old blind man in Treasure of a Blind Swordsman, who deals a Curb-Stomp Battle to Jack and Wuya in a showdown despite not being able to see anything.