—ZUN on Perfect Cherry Blossom, the only Touhou game to have a phantasm stage - a level above the usual extra stage.
"I can see it... My mind-reading third eye will show me your very heart! An image of battle... I think I can use this to make you suffer. [...] Now, this is where it really begins! Go and sleep with a trauma so frightful it will wake you up!"
—Satori Komeiji, Subterranean Animism, stage 4.
As anyone who has played Touhou will tell you, the boss fights are what these games are all about. There are well over 100 boss characters in the series, and you encounter most of them only once, so it's natural that some make a more lasting impression than others. In fanon, they might be famous for their colorful personalities, ridiculous cuteness levels, or Memetic Sex God/Molester status, but for those who have actually played the games, they will always be fondly remembered for a different reason.
She may not be the hardest boss in her game. She may not test your skills to the same extent as the Final Boss, or your patience and determination like the Extra Boss. Beating her may not even earn you much bragging rights. But she will make your life a livingBullet Hellnonetheless. She is the reason many people have never seen the good endings. She has left countless broken hearts and keyboards in her wake. Even hardened shmup players have been known to reach for the bomb key at first sight of her. She is That One Girl!
Which boss exactly qualifies for the title varies from player to player and depends somewhat on your choice of character and shot type, but generally every game has at least one boss that will make you go pichuun again and again long after you've figured her patterns out. You can always count on them to mess up your until-that-point perfect game. They are mostly found around stage 4 or 5.
NOTE: As always, the Final Boss and Wake Up Call Boss cannot be That One Boss without being overly hard by their standards. Please do not add them as examples. Also, to avoid cheapening the title, don't list bosses that are only an example on Lunatic difficulty or because of that one spell card you can cancel out with a single bomb. Remember Bonus Boss is completely banned no matter what. This includes Extra stage mid-bosses too.
The Story of Eastern Wonderland
Marisa Kirisame in her very first appearance as the stage 4 boss. She pulls out four multicolored orbs that all shoot danmaku in addition to the bullets coming from herself. Furthermore, she can cause them to spin around outward in a fashion that is very hard to anticipate and dodge. She becomes much easier when the orbs are destroyed... until she simply recreates them. She is also aggravating due to her tendency to fly down to the bottom of the screen, much closer to the player than most other Touhou bosses, which makes dodging her attacks harder or impossible.
Lotus Land Story
Elly, mainly because of her scythe. She throws it at you in the middle of her patterns, which already require a fair bit of precision, it's slightly homing, and it blocks your shots. If you're playing a shot type that can't get around that it'll be a very long fight.
When playing as Marisa, there's Reimu in stage 4. She is a nightmare to defeat, since she has several attacks that can either spawn very fast spreads of bullets, yin-yangs which are large and bounce off the sides of the play field in erratic fashions, or wall in the player and force them to dodge fast aimed bullets.
Mai and Yuki, the stage 4 bosses, are pretty frustrating due to the sheer amount of bullet spam coming from two different directions, as well as Mai's laser and small bullets. And that's when you fight them together. Depending upon who received the most damage, you fight the survivor, and it gets harder. Mai uses bouncing ice daggers and egg bullets that break into more bullets when your shots hit them. Yuki, on the other hand, just spams slow and fast patterns, but also uses fireballs that block shots.
Yumeko, the penultimate boss of the game, is known for being a nightmare to defeat even for experts. While the difficulty of the game is gradually but steadily raised until her battle, she herself is a nasty difficulty spike. Half of her patterns require very fast reactions - she just throws some knives and bullets. But later, you have to deal with VERY tightly aimed streams of bullets/lasers AND knives she randomly throws everywhere. If you survive that, prepare for some very fast bullet walls followed by more knives... from both sides of the screen! Her final attack, which "only" requires dodging diagonally thrown daggers, seems easy by comparison. And since all of her patterns are speedy and need very good reflexes rather than good bullet trajectory reading, you will usually have to bomb in advance to avoid losing even more resources. Just watch this video...
Right BEFORE Patchouli, we also have Meiling, but her battle might be more of That One Spellcard than anything else, because of that accursed Rainbow Windchime spell.
Perfect Cherry Blossom
The Prismriver Sisters, come at the end of stage 4, which is already arguably the hardest non-extra level in Touhou. Yet again, the cards they use on you depend on your character choice, as well as which one of them you focus your shots on. Their nameless non-spell patterns fill the screen with a truly ridiculous number of bullets on higher difficulties. These are the same every time, so it's possible to find a safe spot and/or memorize them, but if you get hit even once while dodging them, Dynamic Difficulty kicks in and changes up the pattern, making memorization completely impractical.
The main downside to playing as Sakuya is you're forced to fight Merlin at the start of the Prismriver Sisters battle. Why is this a downside? Because Merlin's gimmick is homing lasers!
Once you get past the Prismriver Sisters, there is another difficulty spike awaiting you - Youmu Konpaku. She keeps you company all the way to the final boss as miniboss of stage 5 & 6, but it's her boss battle that's evil. Her favorite method of attack is to throw dense AND rather fast (or just extremely dense) clusterfucks layers of bullets, while giving you about 3 seconds of bullettime to position yourself out of harm's way. In theory. You need a lot of practice to get used to this to the point you won't be reaching for the bomb key during every slowdown. Youmu singlehandely proves that reading-based spells can be just as hard as reaction-based ones. Demonstration.
Reimu uses heavily patterned, but extremely dense clusters of spell tags, fires off powerful homing shots, and boxes you into small areas with her attacks. In the second part of the fight, she's usually zipping all over the screen, making herself nigh-impossible to hit. Not so hard as she was in LLS, but really hard anyways.
On the other hand, Marisa's attacks are much less structured. She prefers to shower the entire screen with her deceptively large looking star bullets. Her first spell card generates spirals of huge stars, showers the field with tiny stars coming in from the sides, and adds periodic shotgun-style blasts from her familiars. Then you get to the secondhalf of the fight...
Phantasmagoria of Flower View
Aya Shameimaru. She's fast as hell, and it doesn't help the AI makes her excessively cheap, dodging almost everything you'll throw to her. Usually you will lose one or two lives trying to defeat her, before the RNG goes easy on you and let you defeat her more easily, but even if you defeat her, you'll have yet to face Komachi / Yuka and Eiki Shiki, which is even worse than Aya in terms of AI cheapness. If the character you're using finds her as your 7th opponent, good luck in trying to clear the game in one credit with that character. You'll need it...
Shoot the Bullet
"Seamless Ceiling of Kinkaku-ji". The nightmarish new impossible request of Kaguya Houraisan is impossible for many a player indeed. It starts as a seemingly doable pattern of falling almost half-screen bullet walls and a spinning layer of bullets thrown around the stage to make it harder to approach her. Said layer gets thicker after each successful photo and reaches a ridiculous level for the last one - it's one of the scenes many people simply don't have the patience to clear.
Mountain of Faith
Aya, from the past main game, returns as the stage 4 boss. The luck factor is vital for that fight, especially the survival card almost at the end.
Parsee Mizuhashi, despite only being a stage 2 boss, has become rather infamous for surprising players with how difficult her spell cards are. "Green-Eyed Monster" is particularly brutal, as she uses it when she encounters the players as a midboss, and it's homing, a quality typically reserved for the bosses of stages 5 and 6 in a Touhou game. Another gimmick not normally seen until endbosses is the Doppelgänger Attack, which Parsee also uses (her "Tongue-Cut Sparrow" spell cards).
Yuugi Hoshiguma in the stage 3 usually trips up players due to the sheer brute force of her attacks. Her final spellcard, "Knockout In Three Steps", is particularly brutal, because it's ensured you will lose at least one life if you don't know exactly what you need to do to pass this spellcard, and if you messed up... *pichuun*. She's often cited as a reason to not assume a character's strength from their placement in the game. One such argument spawned the NOT EVEN DROP meme.
The first spell card she uses, Oni Sign "Anomolies, Strength, Disorder, and Spirits" is rather hard even on Easy. Yuugi shoots out a spiral of danmaku that are facing ever which way, before they go out in haphazard paths. You can get in the way of the spiral to not make those shots appear, but it won't really help.
Satori Komeiji can be this, depending on who your option is, as she uses that character's spell cards. Heaven help you if you're playing as ReimuC and aren't prepared for "Tengu Macro Burst" to come hurtling at you. If you are playing as ReimuA, then have fun with Yukari's "Border of Wave and Particle", which was part of the extra stage of Shoot The Bullet. And then there's MarisaB (Patchouli). Her torturous water cards are the ones that Satori chooses, even if one of them ("Princess Undine"), isn't one of the spellcards that Marisa remembers from her. Well, those and one of Patchouli's cards from the extra stage of EoSD, the "Philosopher's Stone".
Orin is almost universally declared the worst of them all. For beginners, there's the non-spellcards from the cat form in the stage 5 battle. Not so hard as it seems, but you can get clipped easily. Then there's the infamous "Cat's Walk" / "Vengeful Cat Spirit's Erratic Step". If you don't read the card's pattern correctly, well... And then, there's the actual boss battle. The zombie fairies are the least of your worries. Her third spell card, "Needle Mountain of a Former Hell" / "The Needles of Yore and the Vengeful Spirits in Pain", is really hard to read, due to the spinning ghost wheels, which are almost but not entirely unlike any other attack found in the series, and the bombs are useless against the wheels, so have fun.
Her final encounter in Stage 6. For those that played Battle Toads, remember the final part of Stage 3? It's basically that, that is if you care about the spell card bonus.
With this many That One Boss characters, some fans like to joke that Subterranean Animism is That One Game.
Undefined Fantastic Object
Ichirin, together with Unzan, in stage 3. Be prepared to deal with Unzan's fists while you try to dodge the other bullets and lasers.
The Captain Minamitsu Murasa, which is the stage 4 boss also qualifies as weell. Giant anchors, literal raindrop dodging, and a homing survival card, all at stage four. If she doesn't mess you up, then...
Shou Toramaru, the stage 5 boss, definitely will do it. Many people have great difficulty with her, especially with the curvy lasers. Her most notable spell card is Most Valuable Vajra / Vajra of Perfect Buddhism. The spinning homing lasers plus the other bullets make this card insanely difficult. It gives you so little opportunity to damage her, that with certain shot types it basically becomes a survival card too.
Satori's "Vague Recollection of Kinkaku-ji". See that example in Shoot the Bullet section? Satori, being the troll she is, makes the players relive the horror again. This time, you only have to take 3 pictures... because the spell starts in the hardest mode. And the bullet pattern before the last photo is even harder than the original spellcard's hardest phase.
Iku Nagae, the stage 10 boss.note This game has 14 stages, being just a Boss Rush with no actual levels. Though she originated in the fighting game Scarlet Weather Rhapsody, and was not even the final boss in it, practically all her spell cards are a nightmare. Just try and capturing "Pearl of the Five-Clawed Dragon" with Hatate, who makes every other card in the game easier. And then you get to "Dragon Palace Messenger's Swimming Shot", which feels downright broken. No strategy or trick works against its combination of completely random curvy lasers mixed with completely random large orbs with big hitboxes. This is the last scene most people clear, usually by pure luck, if at all.
Yoshika Miyako, the resident jiang-shi of Stage 3, has the annoying gimmick of absorbing spirits that emanate from her body to restore health. If you don't get close to her to intercept the spirits, she can easily deny a spellcard capture, even if you are wailing at her health the entire time. In addition, she fires large, spread out waves of kunai danmaku that can be very difficult to deal with, especially with a forward shooting character. And that's not even getting to the problems Youmu has...
Seija Kijin, the stage 5 boss. Do you remember Sakuya, Youmu and Reisen and how their gimmicks screwed many players? Well... she goes one step further, having the power of turning anything over, and she'll be glad of using this ability against you. She isn't as strong as other bosses in terms of bullet spam, but she doesn't need anything of that because of her ability. She screws you over by flipping the screen horizontally and your controls with it in one spell card, then vertically in the next, then rotates the screen 180 degrees every few seconds in her last spellcard. You'd better have enough lives, because you'll need them.
Megumi Yaobi, the stage 5 boss from the fangame Marine Benefit. She has the gimmick of flooding the screen while giving the player a bubble to stay in; if your hitbox leaves the bubble for even a millisecond, you die. On top of that, she moves constantly all around the screen, making it very hard to actually deal damage to her. For her last nonspell, she even circles the screen while spraying bullets everywhere. And when you think it's over, she pulls out her survival card, The Eight Million Laughing Gods, which is That One Attack to end them all. It's not unheard of to lose two or three lives to this one card.note The latest version nerfs EMLG somewhat, largely removing the threat of the moment which in previous versions was almost guaranteed to kill you, but even after the nerf the card still requires a lot of very precise movement at awkward angles.
As if this weren't enough, she shows up again as the stage 6 midboss, and deliberately goes out of her way to block your path through the bubbles with the biggest projectile available to her - MegumiYaobi. If you aren't prepared for this, and are just watching the bullets and the bubbles while paying no attention to the boss, as is typically expected of the player, you WILL die. And even if you are prepared, the attack is nightmarishly difficult to read, considering how the bullets come from absolutely every direction possible for bullets to come from, all at once, without any pattern to speak of. Arguably the hardest attack in stage 6, and the player hasn't even seen the final boss yet.