Jojos Bizarre Adventure: During Ripple Era a single ripple-powered punch was enough to kill either a zombie or a vampire. The most difficult part was landing the blow, since undead possess abilities exceeding those of human beings, putting this trope in use for both protagonists and antagonists.
The Administration Bureau's ultimate weapon, the Arc-en-Ciel, is also a One Hit Kill by its very nature. Though it looks like a Wave Motion Gun, what it actually does is twist space-time around its target, seal everything within a hundred-kilometer radius into a pocket universe, then allow said pocket universe to Big Crunch itself. The weapon is far more deadly used on planets than in space, due to the vacuum effect that so much suddenly vanished air/land would leave behind.
Urek Mazino all the way. In what little screen time he had up till now in Tower of God, he one shot certified BAMF Viole, killed two Rankers in an instant and took another down with a simple kick. And this is just all the fighting we have seen on screen.
The invaders during the Battle of Mahora arc switched from stripper rays to "time displacement rounds" that send the target three hours forward in the future to when they've already lost for the ultimate in battlefield removal when things got serious.
Asuna's fan also inflicts one hit kills on shikigami and summoned beings. Since she can do thisanyway, she replaced it with a BFS which can do it from a short distance.
it was assumed by the characters that Bartholomew Kuma's main attack was this, until it was eventually revealed that it merely sent its target flying away so fast that it looked like they vanished.
Subverted with Usopp, who uses the term "hissatsu" before every single projectile attack. None of them are ever certain hits, and definitely not kills. Then again, this is him being Genre Savvy, because the enemy is supposed to think they are. Usopp's attacks might not be one hit kills, but he has never missed a target since he joined the Straw Hats. While disguised as Soge King, he used a large slingshot to accurately snipe at and disable marines over a mile away.
Impel Down's Chief Warden Magellan has the ability to produce lethal poison from his body. When he fights, he simply floods the room with it, making him absurdly powerful.
Not a kill, but one-hit nevertheless. While Bellamy's crew, minus the first mate, was utterly horrified that they had just irritated someone with a hundred-million beri bounty, Bellamy himself was convinced that Luffy had just created a false bounty poster to make himself seem more threatening. So, when Luffy comes around, he decides to put that theory to the test. Luffy takes him out with one punch.
Let's not forget Zoro's Single Sword: Lion Song technique, which cinched the battle with Das Bones and is suspected would have taken out Kaku had he not dodged.
A minor character example is King Elizabello the Second, whose King Punch is so powerful it pierced the wall of a fortress, and is said to be capable of bringing down an Emperor. However, charging it makes Elizabello basically helpless, and can only be used once per hour. When we see it in action, it definitely lives up to its reputation: The punch's shockwave extends for a considerable distance all around him (despite it being a punch), this blast alone instantly taking out anyone caught in it regardless of their fighting skill. The only person seen to withstand Elizabello's dreaded King Punch is a Barrier Warrior who saw it coming and prepared accordingly.
It's been summed up in Death Note that no matter what, as long as a name's been successfully written down, the victim can't escape their fate. All in the name of Anyone Can Die. The only variety is in how the death takes place:
The death can be delayed by at most 23 days. In the live-action movie adaptation, this is how L beats Light: he writes his own name in the Death Note with a time of death 23 days later, thwarting the Gambit Roulette that led to Light's victory over L in the anime and manga versions.
Simultaneous (within .06 seconds) use of at least three Death Notes on the same victim will not result in death for either the victim or any of the Death Note holders, within 23 days or otherwise. Practically impossible and never occurs in the storyline, but still there.
Another exception is if the user of a Death Note unintentionally misspells a person's name at least four times, which is far more plausible (particularly with Japanese names, which can have a lot of homophones). The one time it came up, Light was lucky enough to have spelled it right on the first attempt.
The Winged Dragon of Ra, which has an ability that is literally referred to in the Japanese version as "One Turn Kill"
There are various ways to pull off an 'Automatic Victory' in the card game, such as with the infamous Exodia (get all 5 pieces in your hand and you win, period; once that happens there's no possible defense). There was also a very-near One-Hit Kill with Wiraqocha Rasca's anime effect.
In Yu-Gi-Oh! GX, Saiou, the Big Bad of season two, took this a step further, managing something called a "Zero Turn Kill" in one episode, winning the duel before he even got one turn. (The method he used is impossible in the actual game, seeing as he used two cards that only exist in the anime. There are ways that this can theoretically be done in the real game, such as drawing all five of Exodia's components in your opening hand, but it is unlikely.)
One particularly famous example came from Yu-Gi-Oh! 5Ds, where Yusei was facing off against 3 goons who had an amazingly simple and solid effect-damage strategy going against him, dishing out a lot of damage on each of their first turns. On his second turn, however, he proceeds to unleash a combo that allows his Nitro Warrior monster to execute three One Hit Kills in rapid succession. The goons' boss even refers to it (in blatant Engrish) as "One Turn Three Kill."
Yuki Nagato in Haruhi Suzumiya - "Commence termination of data link". Pwned. This is also the reason why she won against Asakura, who was busy jumping around, throwing metal spears, while Yuki hacked the time-space program (whatever that is). It's also why Yuki was so seemingly weak for the bulk of the fight: she was devoting all of her attention to setting a single winning move.
Ranma's encounters with Herb and Saffron ended this way, as he turned their own power against them in devastating variations of the Hiryu Shoten Ha. More notable in the case of Herb, who was knocked out instantly from this one attack (he had been punched in the face earlier, but it hardly even hurt his skin.) In Saffron's case, the technique froze him solid and shattered him.
Ryouga and Ranma both believed that the Bakusai Tenketsu ("Exploding Point-hole," or "Breaking Point" in the English version) is one of these, since it consists of making rock, soil, and (non-living) wood burst into pieces with the touch of a finger. However, after having her fun watching the two teens struggle, Cologne revealed that it doesn't work on living things.
This is why Tohno Shiki from Tsukihime is described as "the greatest wild card" (not the trope): since his Mystic Eyes perceive the concept of Death as lines and dots over everything, all he needs is one chance to kill his opponent. Given his natural killing abilities and inborn assassins' skills, all of his fights begin/end with this. In some scenarios, he has been able to kill hallways, a vampire infection (before it takes over his body), and the poison inside someone else's body; although this requires it to be Cast from Hit Points.
Shiki Ryougi from Kara no Kyoukai has even more hax, she can stab magic to death, like ghosts or telekinetic blasts.
Technically, Tohno Shiki is capable of doing this as well, except Ryougi doesn't seem to have the Cast from Hit Points issue. She also doesn't seem to have the problem of having to understand the nature of something's existence first before she can use her eyes to kill its existence.
This is due to the fact that the two Shiki's eyes work slightly differently. Tohno Shiki's eyes perceive the point of death on something, which when attacked, unravels what makes the object considered to be "alive", and in turn results in death. Ryougi Shiki, on the other hand, perceives the fundamental existence of something, allowing her to kill anything. Ryougi isn't weakened because after her accident, her "male" personality basically "died" and the space it occupied in her consciousness was filled instead by Akasha, the Origin of Existence itself, allowing her to understand everything that exists in the universe, and giving her complete control over her Mystic Eyes.
Psyren has Kyle doing this to Dholaki, by psychically boosting his speed so he can shatter the weak points on his body, killing him almost instantly.
The second summoning of Suzaku and the presence of all Suzaku Seishi notwithstanding, this is how Taka ends the battle with the fake Suzaku inFushigi Yuugi's third OVA, Eikoden. Sure, being able to send an evil being into oblivion with one punch is awesome, but in terms of action, Tamahome's reincarnate doesn't live up to his standards much.
Soi Fon's zanpakutou in bankai produces a nuke. Its special attack is a One Hit Kill loaded with only one hit, and it takes her days to recuperate. Her shikai kills in two hits that occur so fast the victim only feels one. However her bankai being a One-Hit Kill is unfortunately an Informed Ability; when Soi Fon first fires it, her opponent easily avoids it, and while her second shot hits in convincing fashion, it appears to do little to her opponent but make him angry. To be fair to Soi Fon, she did have the misfortune of breaking out her bankai in a battle against the former God-King of all Hollows, whose Story Breaker Power was second to Aizen.
Yamamoto's bankai Zanka no Tachi has at least two attacks that can assure an opponent's demise in one strike: "Rising Sun Edge", which concentrates the flames of the sword upon the edge of his blade, burning to death anyone who gets cut by it, and "Tenchi Kaijin" (Heaven and Earth Reduced to Ashes", or something like that), which sends a concentrated flame/heat slash to incinerate the target immediately (more "kill outright" than "ensure demise").
The Jyuken (Gentle Fist style) is an entire taijutsu form based around one hit kills. Both Hyuuga Hiashi and Hyuuga Neji have killed targets with one well placed strike. Since the Gentle Fist can target the internal organs of a foe directly, a simple jab to the heart is instantly lethal.
Itachi Uchiha's Susanoo wields the Totsuka no Tsurugi, a sword that seals anyone it pierces in an eternal genjutsu. To date, Orochimaru and edo Nagato, two of the strongest shinobis in the world have been subjected to this....treatment.
Hatake Kakashi has the technique Kamui, which can warp away it's targets to an alternate dimension.
Subverted in that most conventional attacks in Naruto that would be a one-hit kill (impalement, shurikens, poison) are typically ineffective due to the haphazard usage of clones, body doubles, illusions, and regeneration.
Don't forget the two most well-known ones: Dragon Slave, which is only beaten by the ultimate definition of OHKILL, the Giga Slave. Then, there's also Ragna Blade, stemming from the same source as Giga Slave, just on a one-to-one basis. Though even before the end of the first story arc, the Dragon Slave has been hit so hard with Serial Escalation that it's almost purely a Worf Barrage.
In Holyland, Chapter 29 mentions that the best way of taking on multiple opponents if you absolutely must is not to go for a decapitation strike by trying to take out the leader or strongest fighter first, but rather to use one hit knockouts to quickly scythe through the lesser fighters. It also carries the benefit of demoralizing the remainders.
In YuYu Hakusho, Kuwabara does this to Risho in the Dark Tournament arc, due to a huge powerup when Yukina arrived. Granted, it was actually two attacks (the first to destroy Risho's Armor of Clay, and the second to launch him across the stadium), but he did them in such quick succession it technically counts as one.
The manga played it straight: Kuwabara simply Megaton Punched Risho, sending him flying and destroying the Armor of Clay at the same time.
In A Certain Magical Index, Fiamma of the Right has "The strike that ends everything it touches", which annihilates anything it hits without any destructive force, meaning it cannot be blocked, and "The strike that reaches everything when swung", which reaches the target without any speed, meaning it cannot be dodged.
In Rosario + Vampire, Akuha Shuzen of Fairy Tale has this ability. Using a magical technique which 'delays her existence by a second' every one of her attacks can just cut through people like paper, taking them apart by basically cutting the reality they're standing in. Three people so far have shown resistance to this attack method, and two of those do it by blocking it with the same technique (the third uses a separate technique designed for the sole purpose of defending against Akuha). Two other people survive it because cutting them in half isn't enough to kill them: One is already dead, and the other simply put the two pieces of her body back together and let her extreme Healing Factor handle the rest.
Ichika's Infinite Stratos is capable of only this, its ability basically amounting to "rush in to disable the opponent."
In principle, this the effect for anything with beam weaponry in the franchise. In the original Mobile Suit Gundam, Zeon held the advantage over the Earth Federation because their Mobile Suits could practically No Sell anything short of battleship beam weaponry (which were too cumbersome to reliably hit a fast-moving mobile suit). Once the Gundam and Guncannon, the first Mobile Suits to use beam weaponry, were made, Zeon was forced into a game of catch-up and saw their numbers quickly dwindle because of the Newtype kid in the beam weapon-carrying Mobile Suit.
In Mobile Suit Gundam Seed Destiny, the Destiny Gundam's Palma Fiocina, or Destiny Finger, is made out to be this. Whether on a basic mobile suit, or the gigantically overpowered Destroy Gundam, if this attack lands, the fight is over. The only time it fails is when it doesn't hit.
In later Universal Century stories, mobile suit designers all but give up on the idea of armor, since even the thickest armor is ineffective against beam weapons, instead focusing on making them as light and agile as possible. When everything can kill you with one hit, after all, the only viable defense is to not get hit.
The Palma Fiocina gets its Fan Nickname because it's a Shout-Out to the Shining Finger and Bakunetsu God Finger attacks in Mobile Fighter G Gundam, which always end the fight in one hit when landed successfully. During the preliminaries of the Gundam Fight, any Gundam that has its head destroyed is eliminated. Thus, the Shining Finger involves grabbing the opposing Gundam's head and melting through it. During the finals, the corrupt Prime Minister Wong Yunfat changes the rules so that a Gundam can be rebuilt after a battle even if its head is destroyed. Cue the Bakunetsu God Finger, which disintegrates the entire enemy Gundam.
This is the premise of One Punch Man, where the protagonist can kill anything in one punch. He finds it extremely frustrating, because his fights are never challenging.