Psyphon turned out to be quite Badass, so props to him too. Especially when he made Armodrillo lose control.
Points to Ben though for electrifying himself in order to make sure he didn't hurt anyone. Seeing how the electric shocks still affected him when he went back to human, that took guts.
Ben has difficulty with using his new alien Gravattack, and is fighting a mech, while still learning about Gravattack's powers. He wins anyway.
Even better? At one point he pushes the mech down to the ground, but he's not worried about that. He's using the gravity powers because he knows the docks won't be able to handle that much mass and thus the mech will sink into the water!
And the best part? When he's about to be attacked by missiles, they don't hit him. They circle around him. Ben says it best.
Gravattack: I have my own orbit? THAT is pretty cool!
And then he throws all of them on his opponent at once.
Young Ben's defeat of Malware as Diamondhead in "Trouble Helix", especially the final dialogue:
Ben finally controlling Alien X and reconstructing the Universe when it is destroyed due to the Annihilarg.
In "Hot Stretch", Ben transforms into NRG and begs not to let the Kraaho remove his suit. They do. And then he proceeds to eat the fusion engine!
Also from "Hot Stretch", Rook's stand against the Kraaho is pretty awesome. He's out of his element, overheating in the Kraaho's own environment, but he never stops fighting.
Villainous one for Khyber in "Of Predators and Prey" for actually beating Ben!
But then that was turned around as Ben and Rook made a plan so that Khyber would deliberately capture Ben and then Rook would follow him to where Khyber was!
Ben taking down Humungousaur's natural predator... as Grey Matter. Utilizing the same nerve pinch Khyber used to subdue Rath.
Despite being stuck as a useless aliens in "Outbreak", Ben uses his knowledge of his forms to help Rook and the other Plumbers take down the baddies, who have been transformed into his aliens.
Max and the plumbers taking down Dr Psychobos robotic worms. At first their shots do nothing, then Blukic and Driba give them improved Ammo, and they destroy the rest of the worms.
Practically every fight involving Ben in "Gone Fishing". After the Badass Decay he suffered in previous episodes, Ben finally shows why he is the saviour of the universe.
The scene in "Malefactor", where Ben overcomes the illusion created by Hypnotick of being surrounded by adoring fans, by reminding himself of what being a hero is really all about.
In "It Was Them", Rook has attained a bomb to take out Dr. Animo's ants, which have him and Ben drastically outnumbered and mildly outmatched. He states that he can detonate the bomb to destroy the giant ant hill they're in, the ants, him and Ben, in addition to several populated areas. Ben shows no shock or alarm until Rook mentions the populated areas. Think about that for a second.
"Ben Again" has several:
Paradox vs. Eon. Bonus points for seeing Paradox angry for once.
The final battle features 11 year old Ben as Articuana and 16 year old Ben as Wildvine holding back Eon, while Old Gwen and Young Gwen preform a spell together to free Paradox.
"Showdown Part 1" might as well have been called "Azmuth is Awesome". To expound trapped with Khyber's hound transformed into the most dangerous predator the Galvan race ever had, he refuses to treatDr. Psychobos as anything more than a nuisance. When he learns that he can't teleport out, he instead teleports Ben and Rook in. After learning the hound is controlled by Khyber's whistleing, he uses Rook's Omnitool and a bone to make a flute, and keeps the hound transforming to disrupt its attack. And then to cap it all off he figures out how to turn the hound into a Cerebrocrustacian's perfect predator and makes Psychobos his bitch. Damn.
There's also 11 year old Ben overloading Malware and nearly killing him.Don't get Ben mad.
The finale of "Showdown Part 2" is a crowning moment of awesome for Ben, Azmuth, the Mechamorph race, Kevin, and Khyber's pet who all help take Malware down once and for all. The real crowner has to be Ben regaining Feedback and Feedback than procceding to drain Malware of all his power.
Vilgax, despite the fact he gets almost no fight scenes in "Vilgax Must Croak", manages to survive the episode, outplay everyone involved, and ultimately wins.
He gets one right at the start of the episode. He's considered so dangerous that Incarceron is cleared out of all other prisoners and staff. He was going to be the only prisoner because they were so afraid of his ability to manipulate others.
Way Big VS Way Bad(A mutated Way Big).
After that, Way Big VS Incursion ship.
Julie with Powered Armor vs Princess Looma. Julie delivers the Knock-Out with her tennis skills.
It's not as noticeable as the rest of the examples on here, but in Store 23 the local Six-Six has placed an arm bomb on Ben 23. Now while Ben freaks out as this, Ben 10 as XLR8 immediately springs into action, using the super speed to run through every possible combination it could have to come off, and with a second to go rushes faster than normal, places the bomb near Six-Six and gets back in time to tell 23 to cover his ears. This is followed by the older Ben taking on both Six-Six and Tetrax at once.
Sandra Tennyson standing up to and stopping Ma Vreedle from blowing up the sun.
Will Harangue getting mutated into an alien freak.
Crashhopper, the Butt Monkey of Ben's alien forms, taking down Psyphon's gang in "Mud Is Thicker Than Water", followed up by taking down the stolen tank Psyphon is using.
Upgrade(Ben) fusing with Kevin's car for a Humongous Mecha battle in "Otto-Motives", beating OTTO, who had fused with the Proto-Truk and 4 rare alien cars.
Argit of all people defeating Ultimate Spidermonkey.
Kyhber revealing that he has a new henchanimal, Ditto's natural predator, a reddish-purple and black cat-like creature that has the same Me's a Crowd ability its prey has. Ben, who had the bad luck of turning into Ditto, manages to stay one step ahead of it for a while.
We finally get to see Atomix, the last of the aliens mentioned in "Ken 10", and he does not dissapoint. A large ham flying brick with energy powers, he takes down both Albedo's Ultimate Humongasaur and Ultimate Spidermonkey forms and makes it look easy.
Word Of God says he is the 2nd strongest alien, after Alien X. Considering Alien X's condition, this seems to indicate how powerful Atomix is and how much we can look forward to see.
Ben is able to control Ghostfreak in the very same episode. Given all trouble he's had with this before it's great to see him be able to finally do it. Even if it's implied to be the chains that Ghostfreak now has on him that's keeping him from going berserk.
Also, Ben taking down a magic golem as a human, since the Omnitrix is stuck on Ghostfreak and he doesn't want to take any chances. He uses his agility to trick it into bringing a giant pillar down on itself.
"Charmed I'm Sure": Swampfire's new form proceeding to take control of Ghostfreak's army of plant-like monsters and fight a superpowered Charmcaster.
Heck, superpowered Charmcaster, complete with golem mech armor. Ben calls it "the coolest thing ever". Then after Charmcaster attacks them:
"Catfight": While it is rather petty and small, Ben fixes a serious situation while in human form by saying Looma and Attea don't have to marry someone to be a good ruler.
Vilgax gets a villainous CMoA in "And Then There Were None"; After spending his previous Omniverse appearances as a secondary and flashback villain respectably, he regains his Big Bad status by successfully wiping out every Omnitrix-wielding Ben (and Gwen) Tennyson and their timeline from existence, including the Ben who has constantly thwarted his plans.
Ben 10K breaking out the most epic alien fusion ever. Atomic-X. Yes, a combo of Atomix and Alien-X.
"And Then There Was Ben". Which has Paradox and No-Omnitrix Ben round up Gwen, Ben 23, Ben Prime and Ben 10K to come back and kick Vilgax's alien ass. Special mention goes to Atomic-X's finger flick of awesomeness and No-Omnitrix Ben kicking Vilgax upside his squid-head. And likely heading for a future as a Badass Normal Plumber agent.
Brainstorm earning his Badass Bookworm status in "The Vengers", taking on the entire Vengers team with his Awesomeness by Analysis, curb stomping them by predicting their tactics, taking advantage of their weaknesses, using the environment against them and figuring out how to put an end to their good name without even trying.
By extension, Ben's own Batman Gambit. He knew if he took himself out of the picture, there would be nothing for the Vengers to do but fuel their clashing egos.
"The Rooters of all Evil" has a lot of moments, but special mention goes to Molly, the Mauve Shirt female Plumber that shows up every so often, getting into a fist fight with Servantis and winning. The icing on the cake is when she taunts him with the same line he used to make fun of her thoughts of revenge when he read her mind.
Even Patelliday gets to stretch his awesome muscles, in case you forgot he's a pretty tough guy.
Phil: Aren't you getting a little long in the tooth?
Patelliday: You tell me.
Ben finally being able to control Alien X and beating the Galactic Gladiator.
Ben using Gutrot to create a pheromone that attracts Way Bads and unleashing the aliens on the Rooter base.
Argit saving the Tennysons' lives from a crazed Kevin in a flashback sequence.
Kevin faking a Face–Heel Turn and destroying Servantis's ability to Mind Rape people and undoing his hold on the Plumber's helpers.
The Plumber Magistrata (AKA, the head Plumber) telling the Rooters off for loosing sight of the Plumber mission and taking their equipment and base while leaving them trapped in the Null Void.
Ben: You may wanna watch out for the Way Bads.
Ben tricking Argit by switching the real Tiffin for the artificial Tiffin in "The Colour of Monkey". He even pulls a Keyser Soze at the end of the episode!
"Vreedlemania" gives us Ma Vreedle making Vilgax, the guy who scammed an Eldritch Abomination, bolt high-speed off of Earth with a simple "Get". Badass and then some.
Ben defeating Mad Ben as Waybig. With Nanomech.
Let's Do The Time War Again has a rematch between Paradox and Eon, aided by Ben 10,000, time travelling motorcycles and one heck of a Stable Time Loop.
And then there's Ben and Skurd, setting her defeat in motion through simple sleight of hand. And Gwen gets a mention here too, since it was her plan.
Final Countdown featured the return of Kundo, Rook's former master and more dangerous than ever. Rook showed off his full potential leading his team and a temporary recruited (later fully inducted) Fistina as a Plumber. For everything that he has done, he has earned his new rank as Magister.
The fact the Plumberbase was a spaceship was pretty cool.
Blukic and Driba saving Azmuth after the First Thinker used his teleporter to narrowly escape being vaporized by Albedo, reconstituting him before it was too late. AND THEY ONLY HAD ABOUT ONE MINUTE!
Azmuth then proceeds to show Albedo why the Galvan call him "First Thinker" and uses a device to render the madman powerless.
Kai, Ester, and a host of other girls both good and bad uniting to save Ben from the Way Bads in the Null Void.
In The End of An Era, a future version of Max retires from the Plumbers for good. His Rank upon retirement: Magistratus! he goes from leader of the earth plumbers, to leader of the whole enchilada. that is nothing short of pure awesomeness and badassery.
A New Dawn is jam-paced full of these:
George Washington was a Plumber, along with Dr. Ben Franklin among others. How we see the mythical cherry tree myth come to life is pure BADASS.
Ben trusted his Omnitrix and it provided with the alien needed to stop Maltruant's Big Bang. Seeing him shapeshift between nearly EVERY alien portrayed with dramatic music was amazing!
To reiterate: Ben Tennyson was instrumental in the creation of the Universe.
There's also how Maltruant is defeated: Ben shoots him with a Big Bang!
Also, seeing the creation of the universe. Amazingest, indeed.
Mildly meta one with Skurd. The team took an idea mainly to sell more toys and made it work very well, with the reveal Skurd's race is the most important one in the universe, in charge of collecting and spreading DNA across the universe.