Ninjas, Suspicious Persons, Ronins, Killers, and generally NPCs that has a face-covering look that roams in the night. Especially the Samurai Hunter in the third game.
4's Online Mode, which introduces you to other people's characters, often wielding fully upgraded swords that can kill you in one hit is firmly in this territory. Thankfully, they are still AI-governed, meaning that tactics that apply to any other enemies are just as viable.
In 2, gaming the sword enhancement system via Save Scumming, careful choices of which attributes you improve on which particular sword, and smart usage of Title Appraisal will get you swords that can absolutely demolish all comers even on the hardest difficulty levels. The third game is very similar in this respect.
Arguably, the Vegetarian title in the third game falls close to this, as it can be abused to unlock everything that can be unlocked from Samurai Points in under 20 minutes, getting 700-plus of them on each 1-minute-fifteen-second "run" through the game.
The forged weapons in the third game. The best weapon you can acquire can have a maximum of 50 Attack (and as low as 30) before upgrades, which is powerful enough that a lot of minor enemies will die in just a couple of hits. If you're smart enough to loot corpses like it's going out of style, you can very easily make weapons with an Attack rating which hovers around 80 or 90 with very little weight. And since the moves available to your character while Dual-Wielding and Ninja Dual-Wielding are entirely unrelated to the individual skills in the weapon, there's nothing stopping you from creating absurdly powerful blades to dual wield.
In 2, dueling style swords have good damage and easily linked attacks (Plus the most prominent sword in the style, Zannkimaru, has insane range), but suffer from high blade stress and difficulty in getting a hit in with their easily blocked blows. The title "God" (gotten by keeping a sword on your person long enough) halfs blade-stress, while the style eventually learns a quick shoulder slam that stops opponents guards and keeps them frozen long enough to hit (it can even be used on recovering foes to infinitely combo them).
Hairando sword, with full move set. Two chainable, unblockable throws that will make you near invincible. Not very exciting, but useful for quickly getting those late night swords with low drops, without much risk of dying. Plus, it's cool to stab your enemy in the heart and then snap their neck whilst flying through the air.
Getting three Big Shot (amplifies Guard Break attacks) charms in a sword, preferably a long one, and stick a Yato-style (a top stance/Jo-Dan style where all but two of its attacks are Guard Breaks), and watch as your character mows down just about anyone, even in Harakiri Mode
In 4, the very first swordfighting style that you start off with has a move called 'Fujinkyaku', which, although drains a good deal of your vitality, is an unblockable low throw that deals massive damage to an opponent. Combined with a sword that has at least one Able-Bodied charm or more (trait that reduces the wear of the weapon) and you're practically unstoppable even in Harakiri mode. Unlocking the move, however, requires that you must have at least slain a large number of Mooks to get to the fifth (and final) level of your style, and, while in that level, parry or counter an opponent's attack while in combat; which can be tricky if you don't time it right.
In 4, it's absurdly easy to start a playthrough on Easy, pick up the Black Mark't sword from Dojima by killing him in the Cemetary, refining/recasting it to 250 attack power, then farming enemies until you hit 1,000 kills, at which point you unlock the "Oddity" title if it's appraised (which gives an additional +250 attack power). Not only does it max out the weapon's attack stats, but it makes higher difficulties much easier (and that's not even counting attached charms).
In 4, Kogure's Kasshin Iai Jutsu, received after defeating him, is an extremely potent style, with some really devastating moves, including a counter, and nice passive abilities that guarantee you will use this style extensively on higher difficulties and breeze through the game on Easy and Normal.
Goddamned Bats: Pickpockets. Draw your sword and inevitably get dragged down in a multi-party melee (due to offending bystanders who has a good chance to be ronins or other samurais), or lose your hard-earned cash.
Ho Yay: Kansuke, a minor NPC in the third game, really likes your muscles.
Moe: Osei in the third game is very much this. So much so that the developers took note of it and included a suit of armor with her picture and name on, Itasha-style in the DLC package. Also, Laura in the fourth game.
Narm: The horrible voice acting in the English versions. Granted, it's supposed to emulate the early low-quality English dubs of Japanese movies.
The fluorescent green and pink kimono the protagonist of 4 wears in the beginning.
Nightmare Fuel: in one of 4's endings, your character and others are boiled alive! Since this is most likely the first ending people get, we don't know what happened to them afterwards. Then we found out what happen to those characters after avoiding the ending the next time. It was implied that they were eaten by the minister and his daughters!
Player Punch: A lot of bad ends are like this. One is in the second game in which The player fails to protect Sayo during the riot at the Amahara festival. The town doctor berates him/her for it and tells them to get out of town, which s/he does. Flash forward to the player sitting on the roadside, crying his/her eyes out while eating a rice ball.