In "Let The Right One In", Damon killing God knows how many vampires on his own as payback for torturing Stefan.
Stefan + deadpan expression + flamethrower = awesome.
When Elena tried to fight off her first vampire in "Unpleasantville" by stabbing him with pencils and attempting to stake him with a mop handle. She may have been rescued by Damon and Stefan in the end, but at least she wasn't helpless like another vampire's girlfriend.
Caroline throwing Damon to the ground in "Brave New World"
In "Memory Lane", Katherine officially becomes a Magnificent Bitch when she reveals she's built up a tolerance to vervain over the last century and a half, and has only been pretending to be weakened for a whole day just to screw with Stefan.
Her final scene in "Plan B," revealing she's known all along that Elena and Stefan were faking their rift thanks to using Jenna as an unknowing informant, after which she makes Jenna stab herself over a long distance, and effortlessly moves on to make another werewolf after Mason's death. Basically, at this point, she's hurt if not outright destroyed every single major character without even breaking a sweat.
In "Kill Or Be Killed", Mason, a werewolf, has Elena by the throat. He tells Caroline he can kill her before Caroline can get to him.
Caroline: I can take you.
Mason: Wanna bet?
Caroline: Yeah. I do. *commence Curb Stomp, leaving Mason on the ground* Told ya.
And the part a minute later where she kills out two cops with guns in about 30 seconds without getting a scratch... Also kinda disturbing...
Lucy gets one in her very first appearance, being the one to defeat Katherine even while staying within the letter of her life debt to her.
Caroline manages to fool Katherine into walking into a trap set for her by the others, then breaks up the fight between Tyler and Matt by pushing them away from each other.
Elijah ripping out two vampire's hearts at the same time, destroying a window with just a handful of coins, compelling Slater to stake himself....anything he does, really.
His Villainous Rescue of Damon by killing four werewolves singlehanded, including punching one in the face so hard that his neck breaks.
He manages to have one in "Daddy Issues" without even appearing on screen when he sends his warlock to save everyone from the werewolves.
He slaps the, as of that point in the show, second oldest (to him) vampire's HEAD. OFF.
In "The Dinner Party", Alaric kills Elijah, but because he and Damon remove the dagger, Elijah comes back and goes after Elena, telling her that their deal is off. She tells him that they're renegotiating, or she'll stab herself and, after Stefan has her drink his blood to heal her, kill herself, leaving Elijah with no doppelganger for the ritual. He calls her bluff, so she really DOES stab herself. And then, once she has his word that her loved ones will be safe, she stabs him with the dagger, killing him.
Katherine's Batman Gambit to trick Damon into killing Elijah, thus freeing her from the tomb.
In "The House Guest", Damon fights off a psychic witch attack with a flamethrower. And earlier in the episode, he makes Katherine genuinely afraid by pointing it at her.
After pining over the completely undeserving Katherine for so long, it's pretty damn satisfying to see Damon coldly reject her attempt to seduce him.
Alaric punching John in the face after the latter scolds him for not telling Jenna
In "The Last Dance", Damon and Bonnie's Batman Gambit to fake Bonnie's death, including telling no one else about it to be sure it's believable.
In "The Last Day", Damon rescues Caroline and Tyler and kills Klaus' witch, then brags to Klaus that he can't do his ceremony, which requires killing a vampire and a werewolf that very night as a witch casts a spell. Klaus calmly reveals that he has spares of all three. Chalk up another Magnificent Bastard.
Also Matt, a human, taking Maddox, a warlock, with a shotgun from behind.
In "End of the Affair", Sheriff Forbes and Tyler rescuing Caroline from her father.
In "Disturbing Behavior", Damon and Caroline's exchange during their fight:
Damon: I'm stronger than you.
Caroline: Yeah, well, I'm angrier than you.
Katherine impersonating Elena to get her hands on Rebecca's necklace, perfectly reversing the Badass Decay she'd suffered since Klaus' arrival.
In "Ghost World", Caroline's Hold the Line against the tomb vampire ghosts to protect Mrs Lockwood, until Bonnie can send them back.
Elena through basically the whole of "Smells Like Teen Spirit", including her pretty convincing fake drunkenness, determination to train herself to fight vampires, and particularly in her exchange with Stefan at the end, in which she says that she still has hope for his humanity and that it makes her strong and not, as he claims, pathetic. Then she stabs him in the stomach, leaving him winded, and walks off with her head held high. The expression on Stefan's face suggests that even he is impressed.
Katherine impersonating Elena through "Homecoming", then getting back up after being stabbed by Mikael and knocking out Klaus' hybrids with a pair of wolfsbane grenades and a cheerful "Ba-boom!"
In "The New Deal", Jeremy's utterly cold and no-nonsense dispatching of the hybrid who almost ran him over.
In "The Ties That Bind", Elena managing to free her wrists from being tied behind her back and smacking Jamie in the face with his own shotgun.
Klaus managing to talk—well, threaten—his way out of being caught in the spell cast by the witch spirits in the old house, thereby getting most of the coffins back.
Elijah coming back and ripping out a hybrid's heart, spooking Klaus in the process.
In "Do Not Go Gentle", Elena calling Esther out on her vile actions and shooting down her every attempt to justify them. For an instant, Esther actually seems chastened and offers Elena an assurance that Jenna is at peace instead of being trapped on the Other Side like any other vampire's spirit.
Rebekah, Kol and Finn being undaggered behind Klaus's back in Bringing out the Dead and going after Klaus.
"This is family business." -Elijah
In season four, Tyler taking his place as alpha of the pack of newly-unsired hybrids.
April telling Liz and the new mayor about how Professor Shane convinced her father to blow up the council followed by "it's time people started telling the truth."
Elena and Jeremy killing Kol. What makes this awesome in and of itself is that Elena is the newest vampire on the show, and Jeremy is just a newbie Hunter. And they took out an Original!
This includes Elena being pinned to the wall through her chest by a banister support. She's actually able to pull her body across the whole thing despite the unfathomable pain it causes.
Bonnie showing just what she can do with her new dark powers, and sealing Klaus (temporarily) in the Gilbert living room to stop him from killing Jeremy and Elena when he finds out what they did to Kol.
One has to admire Klaus completely turning his capture in the Gilbert house completely in his favor and his amusement. Tyler actually arrives to taunt Klaus on his hopelessness… only for the older hybrid to completely mind fuck and Troll Tyler so completely than one can only watch him in almost horrified awe. The man is even more dangerous with his words than with his actions
And in 4x14, it's the return of the show's resident Magnificent Bitch Katherine Pierce/Katerina Petrova, who just waltzes right into Silas' tomb, takes the cure and leaves a dead Jeremy Gilbert and a dying Bonnie Bennett to deal with the imminent return of the world's oldest immortal being.
The ads played their own part in this. Earlier in the season there was an irritating case of Never Trust a Trailer when an appearance by Katherine was hyped up, but turned out to be a hallucination. But this time they gave no hint that she would appear, letting it be twice as much of a surprise.
And then there's the moment we've all been waiting for: Silas' first meeting with Klaus. It is the first time ever in the series that Klaus tries his usual tricks and finds that he has been Out-Gambitted and, more importantly, outmatched physically. The episode ends with Silas stabbing Klaus in the back with Rebekah's white oak stake, snapping it off and leaving Klaus to scream and writhe in sheer pain. And he does it with no back-up, no complicated magic. It shows how dangerous Silas truly is if he can incapacitate freaking Klaus in less then ten seconds flat.
Made even more awesome by the reveal the next episode that Silas hadn't even touched Klaus, that actually he had merely mindfucked him into submission.
In Picture's of You, for those who found Vampire!Elena to be a Scrappy, you have to admit, Bonnie using her expression on her was pretty damn awesome.
Bonnie putting down Silas for good, using the sheer force of her willpower.
Jeremy's Big Damn Heroes moment when he saves Elena from being killed by Kol.
In the season four finale Klaus decapitates a witch with a graduation cap. Yep. Threw it like a ninja star.
And topped off by Elena force-feeding Katherine the cure.
Jeremy Curb Stomping three bullies in the season five premire.
Damon compelling the principal to give Jeremy a light punishment for it.
Katherine managing to get away from both Silas and Jeremy and go on the run once again. Jeremy is a Hunter. Silas is the first immortal being in history. It just proves that while her vampirism gave her benefits, ultimately it's Katherine's cunning that makes her so dangerous.
Just to illustrate how amazing this is: Silas may be the only truly immortal being on the planet, but he lacks the speed and strength of regular vampires. And since Jeremy is a Hunter, that means Silas can't use any of his mind tricks on him. Jeremy went one-on-one with a two-thousand year old immortal, and he nearly won.
Katherine's Big Damn Heroes moment when she shows up to save Jeremy from Silas, shotgun at the ready. It's the first time she's ever used a gun (according to her, anyway), and she downs Silas with three shots.
A villainous one for Qetsiyah. It's not enough that she orchestrated events so that if Silas died as a witch, he would be with her for all eternity. She made the Anchor - the one thing keeping the other side active - Amara herself, aka Silas' one true love. Meaning that if Silas had taken the cure and tried to destroy the other side, he wouldn't have been able to do so, since he could never hurt Amara. She remains immortal, and he stays a witch until his dying day, and then he goes to supernatural purgatory and Qetsiyah. As Damon said: a bona fide genius.
Her dying words certainly count. After all her scheming and two thousand years, Qetsiyah finally has what she's always wanted: Silas with her forever.
Qetsiyah: True love prevails. The universe be damned.
After five seasons, Katherine is finally put down for good. By Stefan the person that she least expected.