Hakuzōsu. Just, Hakuzōsu. If you have ever wondered "what's the best way to annoy the hell out of anyone you're fighting in PvP?", then Hakuzōsu is your guy. His kit revolves entirely around reducing all of your attacks to dealing miserable levels ofScratch Damage while his team laughs at your struggle. His Sheltering Shadow reduces all incoming damage by anywhere between 35 and 50%, direct or indirect, while his Awakening gives him a new passive, which lets him protect the ally with the lowest HP remaining and take all of the ATK inflicted to them. And if you're thinking his kit seems to have been designed with a specific intention in mind, you'd be right. Hakuzōsu was made specifically as a counter to Onikiri, where his absurd levels of Damage Reduction either negates or reduces the chances of an AoE proc happening, while eating the brunt of it like tofu if it does occur. As it turns out, this also makes him the most broken shikigami in the current meta, since he also makes everyone else deal pathetically-weak damage while his team sits behind an obscenely-strong protection. And if that's not enough, his passive gives him up to 60 extra SPD at the beginning of each match, basically laughing in the face of Rocket-Tag Gameplay since he will almost always move first, and the way his kit is usually built, he's also far too resistant to disable with CC. As such, players with Hakuzōsu are all but guaranteed to win in PvP, even if they don't have the damage output to show for it, since with sufficient turtling, the other players will just Rage Quit after several turns of tickling him and his team.
Speaking of horrific SSR power creep, Onikiri seems to have been a result of the developers looking at Tamamo and saying "Yeah, we can do better than that". Simply put, his passive effect is just flat-out unfair. While he simply does absolutely horrendous amounts of damage with just his basic attack, his passive takes this to new heights with free strikes that deal terrifying Splash Damage that bypasses most manners of shielding, or Shōzu's Life-link. When built right and allowed to attack, his first move is a practically-guaranteed win, especially when equipped with Scarlet soul (counter attack on damage), so you'd better be damn sure your shikigami could off him in one move, or it's game over. Disabling him with controlling effects is the only remotely-viable way of shutting him down, which is harder than it sounds since his Evolution bonus is an increase to Effect RES, on top of the defensive buffs granted by his team. As a result, he really should be at the top of this list, as shortly after his release he became effectively the only DPS shikigami anyone bothers using at high levels (some players even use multiple copies, on the basis that if one OP shiki is good, 2 OP shikis is better). It got so bad that (rather than nerfing him, oddly) the developers had to release Hakuzōsunote A defensive shiki optimised to negate single-target damage, which prevents Onikiri's follow-up strikes as a hard counter to him. And even then, he's still a very dangerous foe if you come unprepared.
Less frequently used in PvP than most, but Shoyō is pretty damn terrifying. What makes him powerful is his dreaded and incurableYou Are Already Dead schtick. To top it off, the damage from the curse is immune to Synchronization, making Shōzu useless against him, and both his special skills, one of which casts the curse when sufficiently upgraded, only cost 2 mana points each. In fact, the top positions on certain boss leeaderboards would almost always have him somewhere in the lineup.
After a skill rework, Chin became one of the most OP shikis in the entire game, enough that she absolutely dominates any boss challenge. Chin on her own isn't that big of a deal- when she attacks a target, she gives it 2 stacks of her poison which do HP% damage on the target's turn. However, when her passive reaches the maximum level, any attempt to apply poison stacks to a target that already has the maximum of 5 instead does a massive burst of damage equal to 30% of the target's maximum HP (capped at 280% of Chin's ATK). Now, consider that poison stacks from multiple copies of Chin all count as the same effect and you're beginning to see where this is going. Equip your 2-3 copies of Chin with the Scarlet soul (counterattack on receiving damage, including transferred damage, itself universally considered completely broken), throw in the aforementioned Shouzu to ensure that any hit on any of your shikis will trigger a counterattack from your Chins, and watch enemies just get torn to shreds. While somewhat limited in PvP, when fighting targets with massive amounts of HP like area bosses the Chin-stacking strategy is often literally the only one worth using, as any glance at the damage leaderboards will universally show.
Shiranui is so broken it's not even funny. Shiranui allows all of her teammates who launched a normal attack a 50% chance of doing it again (raised to 100% when fully upgraded) and adds Life Drain to that second attack, which may not sound all that impressive, but when her team has those that deal obscene amounts of damage in one tap, it gets really serious. Doubly so if the damage type is indirect, scales damage to your own shikigami's HP, triggers everytime anyone on their side moves, and hits everyone on your team at once, such as that of Bake-kujira, and lord help you if she herds around more than one of that shikigami. Shiranui herself hits like a truck, possibly twice every turn, and has a passive that seems to have been deliberately cooked up to prevent the previous Game Breaker extraordinaire, Onikiri, from taking her out quickly, in that it gives her a guaranteed Heroic Second Wind that nobody can bypass. This way, she's guaranteed to survive your initial assault regardless of how much damage you deal to her, and unleash a barrage of hits on your team that hurts a lot, especially if she's the last one standing. Really, fighting Shiranui is extremely unpleasant if you lack her hard counters, of which she has virtually none. The most practical way to fight her is to hunker down with Damage Reduction skills, which usually requires yet another Game Breaker, Hakuzōsu, and keep smashing her silly until she finally bites it.
Kuro Doji and Demoniac Shuten Doji are devastating together. Kuro Doji gains free AOE attacks when he loses HP. When his skill is active, Demoniac Shuten Doji damages his teammates on their every turn and boosts their attacks proportionally to HP lost. Thus the more turns Kuro Doji gets, the more health he loses and the stronger he becomes. And while this team-up isn't very useful for prolonged fights, such as encounter bosses (since it's fairly easy for Kuro to get killed), it reigns supreme in any multistage battles, be it Secret Zones, Area Bosses, Netherworld Gate, or even Ultimate Orochi.
Never mind Kuro Doji, SP Shuten is completely and utterly busted entirely on his own. His Raging Blaze pushes back the action bars of the entire enemy team like Kamikui does (only without damaging them, preventing the possibility of triggering counterattacks), and although he can only do it every few turns until his Demoniac state ends, once is quite enough. One of the most popular PvP teams (because it's completely and utterly degenerate and cheap) uses a fast Yamausagi to pull the already fast Shuten, Shuten's Raging Blaze to push back the enemy team, then the infamously broken Kiyohime+Snow Spirit to freeze them all solid, together with an orb provider to power it all and a single DPS (although Shuten offers powerful supplemental damage with his enhanced Torch The Sky attack even when SPD built). The end result is that the opponent never gets a single action whatsoever, regardless of whether they get blown off the field or slowly worn down. Even Hakuzosu is no protection, as his damage redirection ability only works when he's not CC'd. And if you were thinking that the old standby of a CC Higanbana can save you, think again- not only is SP Shuten completely immune to all forms of CC (apart from pushback and seal, which is the only way to actually disable his CC immunity), his Raging Blaze also extends this same immunity to his own team (to be precise, they can still be affected by CC- it just gets instantly cleansed at the start of their turn).
And then we get to Monumental Otakemaru. SSR Otakemaru wasn't quite broken enough to completely take over the game like Onikiri once did, so the developers rectified it with his utterly broken SP form who, like SSR Onikiri before him, doesn't even put up the tiniest pretence of being fair or balanced in any way. Both his basic attack and (when in Soul Controlling form) his devastating AoE attack apply his special debuff, Mountain's Erosion, which reduces the damage and healing received by his target and then after they act it converts to Mountain Crush, which automatically dazes them until the start of their next turn (not preventing them from acting, but preventing them from using any passive trait that requires them to not be crowd controlled e.g counter attacking) and also reduces their SPD by 30%, delaying their turn and lengthening the period that they're incapacitated for. This has no effect chance, which means it's a guaranteed effect that cannot be resisted by anything other than total CC immunity, which is a very rare trait that very few shikis can gain... of which SP Otakemaru is one. His Master of the Mountain puts him into Soul Controlling form (and when maxed out, causes him to start in this form) which makes him almost immune to CC thanks to its 100% Effect RES boost, as well as (when upgraded) granting him a 50% damage reduction from all single-target attacks, causing every allied skill use to reduce the cost of his special skill by 1 orb, effectively neutering the high orb cost that is its only drawback, and causing him to heal for 30% of his health whenever a non-summoned unit on either side dies, making him extremely difficult to kill. And once 3 units on either side die, he transforms into his utterly ridiculous Reincarnation form, purging all effects from him, making him completely immune to all CC, granting him an insane +100 SPD, 50% reduction of ALL damage, and causing him to automatically unlease a ludicrously powerful skill called Obliterate Evils, which deals 5 randomly-distributed hits to enemies that strike for 240% of his ATK each on his turn, for free. Hell, by this point his special attack skill, Forgotten End, is basically a formality- it "merely" does (when maxed) 240% of his ATK to all enemies after dispelling one of their buffs first (and being very cheap or even free if he stacked up enough Mountain's Boon stacks from his allies' skills while he was in Soul Controlling form). And just to ice the cake, he defies the usual pattern of SP versions of shikis being a Glass Cannonnote due to not being able to be evolved most prior SP shikis have high offensive stats but comparatively low HP and DEF by having straight S and A rank stats across the board, including a ridiculously high (for a mass-damage nuker shiki) 115 base SPD (compared to his Mighty Glacier SSR self, which is one of the slowest shikis in the game). SP Otakemaru was another fine example of the developers getting bored and playing "let's see who can come up with the most broken shikigami possible".
The Hari-onna (Seductress) whose 4-set effect causes a 40% (quite high) chance of extra damage 10% of the target's HP upon crit, thus greatly increases the strength of attacker-types, especially those who attack repeatedly, and one of the most frequently used.
The Sō (Scarlet) whose basic Counter-Attack 4-set effect doesn't sound like much, but can actually be used to horrifying effect on attackers whose basic attacks hit multiple times or have powerful side effects attached.
The Kiryō Utahime (Ghostly Songstress) is the scariest among the special 2-set ones; these give a powerfulArmor-Piercing Attack every 5 hits and thus make any repeated attackers a force to reckon with (especially when combined with Seductress). On the other hand, being a boss soul means it's restricted to PvE play, so this is much less of a problem.