Buffy: If I was at full Slayer power, I'd be punning right about now.
In the episode "Anne", Buffy has run away from home and changed her name to Anne (her middle name). In the course of the episode, she finds herself trapped in a demonic dimension where humans are made into slave labor. One of the demons is trying to break the humans' spirit by asking them "Who are you" and hitting them if they answer anything but "No one". What does Buffy answers, just before kicking everyone's ass?
Buffy:(cheerfully) I'm Buffy, the Vampire Slayer. And you are?.
When Buffy is gone, Willow tries to replicate her punning — doing it herself between Seasons 2 and 3 (when Buffy was in L.A.), and programming it into the Buffybot between Seasons 5 and 6 (when Buffy was dead). With minimal success: "That'll put marzipan on your pie plate, bingo!"
Which brings us to the most cold-blooded pre-killing line of all time: "Bored now." Which was made more poignant by the fact that it was Vampire Willow's catchphrase, last heard 3 years before.
"Bored" was actually a running theme for pre-asskicking. Buffy delivered it at least once (to Dracula), and Glory did too (to Buffybot).
Also this to a nameless vampire:
Buffy: Wow, never heard that one. Okay, how about, "Oh, God, my leg, my leg?"
Vampire: Oh, God, my leg!
Buffy: See, now we're communicating.
Angel attempted a ton of these. One of the few that stuck was in the series finale. Angel is being tossed around humiliatingly by Marcus Hamilton, a consummate Badass, who takes time to gloat over this, explaining that Angel cannot defeat him, that the power of The Big Bad flows in his blood. Cut to Angel switching to his Game Face.
One of the most memorable one-liners from Angel himself has to be from the first episode: Russell Winters, a vampire, gloats about the protection he gets from Wolfram & Hart, claiming that he can do anything now. Then Angel puts his feet on the chair, gets closer and says:
Angel: You can do anything? (pause) Can you fly? (literally kicks him out the window)
On Community, during the second paintball game, a third City College stormtrooper runs up to the two celebrating that they've won. It's actually Pierce in disguise, and when asked who he is, his only reply is "Your mother's lover!" before opening fire and winning the game.
In the first Paintball game they take down the chess club with "Checkmate, bitches!"
I Dream of Jeannie: Jeannie in the climax of the 1991 made-for-TV movie I Still Dream of Jeannie. (She comes to her son Tony Jr.'s rescue by blasting open a set of heavy-duty fire-rated steel doors.). The REAL Jeannie — The Monster, The Creature, The Entity — comes out to play in this scene.
Jeannie: (creepy reverb) DO NOT TOUCH HIM!
Possibly the most emphatic of all. In the finale of the second series of Primeval, after Cutter is being played with by a Future Predator:
Cutter: Enough. *proceeds to shoot Future Predator*
Cutter gets a lot of these.
Cutter: *when faced with giant mer creature* Time to go home.
At the end of the first series of Merlin, this gem.
Merlin: *Gets up from normally deadly injury* You should not have killed my friend. *And then proceeds to harness lightning by merely lifting his hand (in the Merlinverse, powerful magic requires incantation)*
Janeway:(to a blustering Kazon captain) You know, I'm an easy person to get along with most of the time. But I don't like threats, and I don't like bullies, and I don't like YOU.
"Hello. I'm Captain Kathryn Janeway. Welcome to the bridge." immediately before the self-destruct detonated and blew up a parallel universe duplicate of Voyager
Star Trek: Deep Space Nine. Both used and spoofed in the episode "To the Death", where the crew of the Defiant team up with the Jem'Hadar to track down some renegades.
Jem'Hadar First:(to his Mooks) I am First Omet'iklan, and I am dead. As of this moment, we are all dead. We go into battle to reclaim our lives. This we do gladly, for we are Jem'Hadar. Remember: Victory is Life! Chief O'Brien:(to his Redshirts)I am Chief Miles Edward O'Brien. I am very much alive, and I intend to stay that way.
From the same series, Meteor's "Your fate is mine to decide." And an awesome variant in his final battle against The Dragon near the end of the series: Leo tells him of his Undying Loyalty to Gamou, that he is willing to give his life to see Gamou's plan completed, and Meteor, in a reversal of the heroes showing some respect to this sort of villain, says that he'll never be beaten by someone who doesn't care about himself and only lives for another person's goal. "My fate is mine to decide," he says.
From Kamen Rider Double: "Now, count up your sins." Used by Double but coined by Shotaro Hidari's mentor Sokichi Narumi (alias Kamen Rider Skull) Kamen Rider Accel from the same series has what's partially this and partially Pre-Mortem One-Liner: after using the Finishing Move but before the Monster of the Week goes 'boom,' he'll say what literally comes out to "Despair is your goal," with "Goal" in Gratuitous English. (Loses something for English-speakers: Accel is bike-themed, but nobody calls the finish line of a race a "goal" as if it was basketball or something. One sub group got closer to the intent of it with a less literal "Despair is waiting for you at the finish line.")
Parodied, then discussed in Power Rangers RPM at the start of the "Ranger Red" episode, where Scott and Ziggy comes across a group of Grinders terrorizing a mother and her baby.
Scott: Shh... You'll wake the baby. Ziggy: Yeah, and if you wake the baby, I might just have to sing you all a sweet little lullaby, and then tuck you into bed without any dessert, or even a single goodnight kiss... (notices Scott staring at him) Oh, wait, wait, what am I even saying. That... that didn't even make sense. I'm really sorry, um... kinda new at being a Power Ranger. I'm, I'm still working on my, uh... my hero one-liners, so... Scott: Look, Zig. I told you. They're called 'one-liners' for a reason. We're action heroes. When it comes to talking — (Scott takes out three Grinders with a Capoiera kick) — less is more.
Boardwalk Empire: more of a Pre-Fingorn One Liner—Chalky sits down with a Klansman and tells him a story about how his carpenter father was asked to build a bookcase by a white man, and after he worked long and hard on it, the man and his friends rewarded him for his troubles by lynching him. Chalky then unrolls a set of his father's tools, and the Klansman nervously asks what he intends to do with them. Chalky just replies "Well...I ain't buildin' no bookcase."
SG1's Jack O'Neill and Atlantis's John Sheppard, both of whom are masters of this trope, with too many examples to mention. And the rest of their teams are no slouches either.
From Enos: A cop pops out of hiding and pulls a gun on two thugs, saying, "Your tax dollars at work."