Any time people gather with the goal of hitting each other until one of them falls over, the ultimate trump card would be a weapon, spell, technique, trick or what-have-you that makes people fall over immediately, without all that annoying strategizing and healing and stuff. The One Hit Kill is that trump card.
Often, the One Hit Kill isn't humble enough to just kill, instead petrifying, permanently polymorphing, or banishing its victim to the Phantom Zone. At times, it's not even content with a single target, becoming a One Hit Poly Kill.
Usually, the One Hit Kill comes at a price. Sometimes it's a Dangerous Forbidden Technique which to use requires Casting From Hit Points, other times it's just really hard to pull off, or leaves you wide open for a counterattack. One-Hit Kills are a common spell in RPGs. In vide RPGs, it's frequently a Useless Useful Spell because of low hit rates, inability to affect bosses, or a prohibitive cost required to cast it, if not all three combined. Tabletop RPGs, by contrast, have a history of letting those pesky spellcasters make these spells insufficiently useless, turning many fights into a game of "who gets insta-killed first". In martial arts, it frequently takes the form of the Touch of Death. If it only works at the start of a fight, it's a Back Stab. If it only works at the end of a fight, it's a Finishing Move. If every enemy attack is a one-hit kill, the protagonist is a One-Hit-Point Wonder.
There's also a non-lethal variant of this: The One-Hit Knockout. One Hit KO's usually don't involve anything like forbidden techniques or rare technology, just an immense amount of force applied at once, usually in the form of a punch to the face. (Bonus points if a particularly fleshy or swift and satisfying sound effect accompanies the blow.) The drawback of this is somewhat self-explanatory: It's not a kill, only a knockout. Still, it give the user at least a solid 5 minute head-start, perhaps even longer, to make an escape or thwart some evil plans before the target can recover and figure out what hit him. In certain works, this is a common reprisal when someone (accidentally or intentionally) presses someone's Berserk Button.
With bladed weapons, cinematic representation of this trope often becomes a Single-Stroke Battle. See also Chunky Salsa Rule. Often a Death or Glory Attack. Compare Coup de Grâce.
Marv from Sin City never seems to have to hit a person a second time (although sometimes he does anyway).
In The Death of Spider-Man, the Human Torch does this to the Green Goblin. Keep in mind, that in this universe, the Goblin is a nigh-invincible hulking beast with the ability to throw fireballs and not a guy in a suit as he is in most media.
Though this then shifts into Broke Your Arm Punching Out Cthulhu, when Goblin - who's fire powered in the Ultimate universe - comes back from this bigger and stronger than ever, allowing him to fulfill the name of the arc.
Later in that same arc, Peter Parker shows up, bleeding from a gunshot wound, and takes the Vulture out for a while with one web.
In Uncanny Avengers, Rogue accidentally does this to Grim Reaper after absorbing Wonder Man's strength. She was unprepared for just how much more powerful she'd become, and ended up snapping the villain's neck with one punch.
La Quete De L Oiseau Du Temps, this is a favorite of legendary warrior Le Rige, who tends to kill his preys with just one swift and powerful blow of his axe.
Naruto gets to deal one of these out to Sasuke in the Alternate History fic Team 8 during the Chuunin Exam preliminary battles (it's a knockout instead of a kill). It's fully explained in author's notes later on, given the context of the fight—Sasuke was still recovering from Orochimaru's seal being applied (and then removed), Naruto just watched Neji beat Hinata half to death in their preliminary battle and was understandably pissed off, and Sasuke crossed a line he should not have crossed:
Both ways. Fire the laser at a target, the target dies. Single proton torpedo to the thermal exhaust port, the Death Star dies.
Snatch has "One Punch Mickey", Gypsy bare-knuckle boxing champion. Bonus points for doing it when he shouldn't.
Godzilla possesses the "Spiral Ray" in a few of his later incarnations, a supercharged, red/orange-colored variant of his normal light-blue atomic Breath Weapon. It proved capable of annihilating some of Godzilla's most powerful foes ever (such as Super Mechagodzilla, Spacegodzilla, and Kaiser Ghidorah) within seconds. The one and only time it did not provide an immediate victory was when used against Destoroyah - and that was when it was being used for the duration of the entire climactic battle - which demonstrated how amazingly dangerous Destoroyah really was.
Of course, the lethality of the weapon is subverted when used against Godzilla himself twice. The first time, Kiryu had gone berserk from hearing Godzilla's roar and was rampaging across Tokyo before they could fire the weapon. And, the second time, the Absolute-Zero Cannon was badly damaged and could only very temporarily trap Godzilla in ice.
Kick-to-the-face in the trailer to Ong Bak is possibly the best example ever.
In Diggstown, "Honey" Roy Palmer gets pissed off and knocks out an opponent with a single punch immediately after the round starts.
A staple of Choose Your Own Adventure books are the no-escape dead ends, though gamebooks (e.g. Lone Wolf) fit the trope best with instant death if you're lacking the right skill/item or just plain unlucky — being not only accepted, but expected.
In Lone Wolf, the bow and arrow, far from being just annoying, often allows a one-hit kill for the hero, if he chooses the right target and is enough of a good shot (though some monsters are utterly immune to this).
The best Choose Your Own Adventures were better known for their creatively gory endings to your life than for their successful endings.
In the second series of The Chronicles of Amber, Merlin knows a "neat little cardiac arrest spell." He only uses it once, but he just obliterates a Jabberwock with it. Apparently, he has a separate "death spell," but he never uses it. However, he does note that the first spell wouldn't have been a One Hit Kill on a fire angel, as they have three hearts.
In Michael Moorcock's novel "The Eternal Champion", when the Eldren use their ultimate weapon a high-powered energy gun against their opponents, who are using classic Middle Ages armor, their attack is a One Hit Poly Kill, obliterating entire lines of their opponents each shot.
The Ivory Knife in P.C. Hodgell's Chronicles of the Kencyrath, a mere scratch from which will kill; it is described as "the very tooth of Death". As one might imagine, this can be somewhat dangerous for the wielder as well. Heroine Jame carries it in her boot sheath for a long while, at first because she doesn't realize what it is, then after she knows, because she doesn't have anywhere safe she can leave it.
Balefire in The Wheel of Time instantly erases from existence anyone it hits retroactively. And it kills in such a way that even the Dark One (who has power over death and can reincarnate people) can't save them.
Balefire comes at a very high price, though. Since it kills retroactively, the past will be changed - any actions performed by the killed within for everything for a minute back to days (all according to the power of the Balefire) will now not have happened. If this happens to much, reality itself unravels, causing a Temporal Paradox that can destroy the world.
The demon-in-the-form-of-a-sword "Stormbringer" in Michael Moorcock's Elric of Melnibone series kills any non-magically protected human in one blow (it also eats the victim's soul). "Mournblade" is a similar demonic sword, and one story reveals that there is an entire race of these demons — all of them taking the form of swords.
"Morganti" weapons in Dragaera are all one-hit killers. They also destroy the soul of the victim, making revivification impossible, and also making it impossible for the victim to travel the Paths of The Dead to the afterworld.
Any of several magic words in the Inheritance Cycle which cause instant death to whoever they were directed at.
In Fred Saberhagen's Book of Swords series, Farslayer can kill anyone (including demons and gods) anywhere as long as they don't have Shieldbreaker or Woundhealer to save them.
In The Chronicles of Narnia, Jadis uses the deplorable word and basically one-shots an entire planet, resulting in no more subjects for her to rule. But that's okay, a pair of children help save the day on Earth and Narnia
Shardblades from The Stormlight Archive cut the soul of living things rather than the flesh, and cut straight through almost any inanimate matter without any effort. Slicing the blade through one of a victim's vital organs kills them instantly. Slicing it through a limb "kills" that limb, rending it permanently paralyzed and numb. The only beings that are likely to give somebody with a Shardblade a problem are those with a suit of Shardplate, (which can withstand a Shardblade, at least for a while) or beasts so large that the blade can't easily be sliced into their vital organs (and these are BFSs an average of six feet long, so the fact that there ARE beasts that big, and a lot of them, says something about this world).
In The Destroyer book series, the martial art of Sinanju is mostly one-hit kill moves.
In Billy Budd, the eponymous character is accused of conspiracy and mutiny aboard a British man-of-war by the ship's master-at-arms, who is doing this to fight off his "strange attraction" to the androgynous sailor. Upon hearing this, Billy freezes up, unable to say anything in his defense. Eventually, he answers the only way he can - by punching the liar in the temple. The man dies on the spot. Unfortunately, Billy is found guilty of murder by the tribunal of officers, despite them all being sympathetic to the boy, and hung the next day. Interestingly, the tribunal was about to find him not guilty as a weird case of self-defense, when The Captain intervened and convinced them of the necessity of the execution.
In the Star Carrier series H'rulka weapons easily one-shot smaller Confederation ships, being as how their ammunition consists of what amounts to miniature black holes. They also outrange most Confederation weapons.
In Smallville, especially early seasons, there are very few opponents that Clark doesn't take out in one hit.
Game Shows: In its simplest sense, the one-hit kill was a contestant winning the game outright on the first play of the game, before his opponents even have a chance to play. Although rare, the most notable examples were:
Tic-Tac-Dough: The "red box" "Bonus Category," which immediately allowed the contestant another turn if he/she answered a three-part question correctly. The categories were moved to other boxes after the question, and the "Bonus Category" always appeared in an adjacent box or other space that allowed him/her to set up and eventually complete a tic-tac-toe. Played correctly – and it often was – a tic-tac-toe was completed without the opponent getting a chance to play (although he/she was always invited back to play the next game). Eventually, in the interest of fair play, the category was retired, in lieu of the modified "Double-or-Nothing."
Spinning three jokers on a single spin automatically won the game for that contestant ... provided he/she correctly answered a question. While it has happened several times on the game's first spin, only once – the first time it happened – did it result in the opponent not getting to play. (The opponent, a challenger, forced his smile as he was on and off the show in roughly 90 seconds.) The ""first-spin triple joker" quick kill was averted thereafter by allowing the challenger – who always played second – an opportunity to catch up by continuing to answer questions until they either caught up, won by surpassing the opponent's score or giving an incorrect answer; however, the game would end if the challenger spun the triple joker and answered the question correctly.
Like its sister show Tic Tac Dough, the 1977-1986 syndicated version had special scoring categories, which could be used on the player's first turn to build a huge lead and virtually assure themselves a win, simply by severely pressuring the opponent. Categories fitting this example were "Fast Forward" and "Bid," both which allowed contestants to answer multiple questions in a single turn until they either stopped and kept their winnings ("Fast Forward") or completed the terms of their bid to earn the cash ("Bid").
Concentration: Solving the puzzle after making a match on the game's first turn.
In the finale of Life, Crews kills Roman with one blow to the throat.
In the competitive robot combat show Robot Wars robots could be battered into submission and some robots were in fact powerful enough to disable an opponent with a single lucky blow. Robots could also be flipped over and those with no way to right themselves were immobilized and defeated. However there was only one guaranteed way to defeat an enemy robot in a single strike: throwing it over the wall and out of the arena. The first time it happened, no one could believe their eyes. Not to mention, the pit.
The Zat'nik'tel in Stargate SG-1. Although it takes two shots to kill, it only takes one to win.
Done on CSI: NY, where a martial arts expert killed the victim of the week with a single blow to the back of the neck.
It's pretty common in North America for a pro wrestler's Finishing Move to be used as a One Hit Kill. There are many, many matches where one participant spends the whole match getting the snot beaten out of him, then seizes an opportunity to hit his finisher and win the match. Note that this is less common in Mexico, Japan, and Europe.
The One Hit Kill effect is essentially what made Diamond Dallas Page so dangerous in WCW. You could beat him up all day, but all he had to do was hit the Diamond Cutter and he'd come out on top — and he could hit the Diamond Cutter at a moment's notice, from fifty-two different positions.
The Tombstone Piledriver: In the 2 decades that The Undertaker has been using it, you can count the amount of people who have even kicked out of it once on your fingers. In fact, when Shawn Michaels kicked out of it at WrestleMania XXV, the look on Undertaker's face was something akin to "WHAT IN THE HOLY HELL OF FUCKNESS JUST HAPPENED?!?!"
A simple roundhouse kick to the head is a staple of the Japanese shoot-style finishing offensive. It's credible enough to pass for a real KO, and the Mixed Martial Arts world has taught us that it's indeed as powerful.
In Sluggy Freelance, the talking sword Chaz can kill pretty much anything with a single swing. The only limitations shown are that, for certain creatures, a specific part of the body must be hit, and that Chaz only has this power after bathing in the blood of the innocent.
Later, 4U City's military is shown using weaponized teleporters which transport whatever they hit into Another Dimension. Not really a one-hit kill, but definitely a one-hit victory.
The Golden Arrows from Slightly Damned tend to take football sized chunks out of their targets. It's not a guaranteed kill, but it's pretty close.
SCP Foundation: SCP-544 perceived threads extending upwards from the heads of others, as well as from other living things, and could kill anything instantly by cutting its thread with his mind.
Season 1 of Red vs. Blue has Donutthrow aplasma grenade and oneshot Tex, who was halfway across a canyon and in a tank. This gets a Call Back in season 10, where he manages to lob three plasma grenades in one throw to kill three soldiers.
Avatar: The Last Airbender gives the impression that the advanced Firebending ability to shoot lightning is a One Hit Kill if it manages to get a direct hit. This is offset by the difficulty of shooting lightning in the first place (it requires a mindset which is opposite to the usual for Firebending, and insufficient skill is likely to cause it to explode in the users face), and even when mastered the time it takes to build up the charge tends to telegraph the attack.
In the grand finale, Aang's use of 'Soul Bending' borders on a peculiar sort of Have-a-Heart instant neutralization. Ty Lee was feared enough for her ability to -temporarily- kill someone's Bending. Not a 'kill', but in a world where children can throw fireballs it leaves you pretty helpless (Various big name mundanes aside)
The Loch Ness Monster gets one on Bigfoot during their Celebrity Deathmatch fight when he cuts the fight short by slicing Bigfoot in half.
In Spice Girls vs. Hanson, Marilyn Manson gets one on both combatants by crushing them under the light rigging.
In Kid Rock vs. Eminem, Joe C jumps into the ring and eviscerates guest referee Carson Daly, but not before Carson kicks him into the air. He lands on Eminem, and takes control of Eminem's body, which he uses to take out Kid Rock with one hit.
Jean-Claude Van Damme cuts Chuck Norris in half with one hit.
There is possibly a phenomenon where the right type of blow can produce hydrostatic shock, however the jury's still out on whether or not its real.
Chances of an instant knockout are significant if any highly trained fighter is given a reason to go all out against an opponent with little or no conditioning. Even if the attacker has no training, (un)lucky hits can lethally aggravate a pre-existing condition (aneurysms are particularly infamous) and kill in short order.
It is very possible, if very rare, to cause someone to go into fibrillation (heart beating so rapidly and without any rhythm, such that it doesn't actually pump any blood) simply from hitting them in the right place, with the right force, at the right point of a heartbeat. It doesn't even have to be a hard hit. It is unfortunately common in athletes. One minute they're active, then they get what seems to be a hard bump to the chest, and then they're down and out. Of course, CPR and defibrillation, as well as quick medical treatment has a chance of bringing them back. But for all intents and purposes, until their heartbeat goes back to normal, they are dead.
Most weapons, on the other hand, can easily kill with a single hit. That's what they're for.
Of course, after living on a continent of death, hopefully Australians have started to evolve just enough to survive the types of instant death their land and seas so famously have in abundance.
The poison dart frog poison kills even faster; less than 3 seconds; it has to get in the blood though, so contact itself probably won't kill you. Unless it's the Golden Poison Dart Frog, which can.
In naval warfare, a well-aimed or lucky hit in the opposing ship's magazine can lead to that ship being blown up by its own ammunition. This is what basically happened to three British battlecruisers at the battle of Jutland in 1916 and to the HMS Hood in 1941. Similarly, hydrogen-filled Zeppelins in World War One were easily downed with just one burst of incendiary ammunition and a number of World War II planes were quite vulnerable to hits in their fuel tanks, most notoriously the Mitsubishi "Betty" aka the "one-shot lighter" or "flying zippo".
This can also happen with torpedoes. Specifically, a magnetic (or other proximity) torpedo detonating a short distance below the keel of a vessel has the potential to "break its back". Causing the ship to split in two and sink relatively rapidly. It doesn't even have to be in the main magazines! At least one of the hits at Jutland was a turret hit, which wouldn't have been a problem except that the Royal Navy battlecruisers' gunners were bypassing safety protocols for the sake of rapid fire and leaving a nice, flammable chain of cordite leading all the way to the magazines. That, and the fact that the cordite stock hadn't been sufficiently rotated and the stuff was a bit old and chemically twitchy, added up to cause disaster.
In World War I the SMS Szent István was torpedoed in the aft boiler, killing the power needed to use the pumps and throw the water out. In fact a direct hit from a well-placed torpedo will sink most warships, with only the biggest capable of surviving.
Masutatsu Oyama, founder of Kyokushinaki Karate, was considered in his time the embodiment of "One Strike, Certain Death". Three times in exhibitions, he killed a full-grown bull with a single punch.