The Watatsuki sisters and the Lunarians are an interesting case. They are powerful indeed but most of them are extremely unlikeable goons who are so prideful and narrow-minded that they sure as heck didn't see the events of LoLK coming. They're so frightened by the concept of impurity that it turns them into complete wimps to the point it reaches comical levels. Meanwhile, the Watatsuki sisters, the heroes (anti-villains at worst) of SSiB are indeed portrayed as very powerful beings but they were outsmarted by what is possibly a genuine Mary Sue. Also, for so-called Mary Sues, the sisters screws up by allowing a guest to live on their world even though they shouldn't, their so-called hatred of Earth doesn't override their ability to visit their mentor and they both have displayed personality traits that shows they are flawed.
Reimu has many attributes of a Mary Sue but her personality quickly withers that. She's capable of being very stubborn, reckless, selfish, violent and while she's capable of great feats as a shrine maiden, the spell card rule means a certain level of balance is maintained in the story unless the situation requires her to take drastic measures. She has numerous qualities as well making her actually a fairly balanced character.
Yukari is bar none the closest we have as a Mary Sue, if she flat out isn't one. She's typically able to do as she pleases, she outsmarts everyone, including the so-called genius of the Moon during the conclusion of CiLR, she is a blatant mouthpiece for ZUN at times, so far she hasn't really got any comeuppance for any of her dickery and the only time we seem to get a reaction from her is when Kasen tells her they're not allies
and that, ultimately doesn't cause her any real issues except, just maybe, emotional ones if she's capable of that. The only flaws I can think of when it comes to her is possibly laziness and being possibly what a normal human being would consider to be an unredeemable monster but the latter would be more based on speculations. Her most redeeming quality seems to be that she likes to go after extremely arrogant people to the point it seems to be something that genuine gets on her nerves, to the point I wonder if the first Lunar War wasn't to humble the youkai through a humiliating defeat.
I honestly expect her to outsmart Reimu in the next chapter of FS or something along those lines. It would be refreshing if it didn't happen to be the case.
Yukari is bar none the closest we have as a Mary Sue, if she flat out isn't one. She's typically able to do as she pleases, she outsmarts everyone, including the so-called genius of the Moon during the conclusion of CiLR, she is a blatant mouthpiece for ZUN at times, so far she hasn't really got any comeuppance for any of her dickery and the only time we seem to get a reaction from her is when Kasen tells her they're not allies
and that, ultimately doesn't cause her any real issues except, just maybe, emotional ones if she's capable of that. The only flaws I can think of when it comes to her is possibly laziness and being possibly what a normal human being would consider to be an unredeemable monster but the latter would be more based on speculations. Her most redeeming quality seems to be that she likes to go after extremely arrogant people to the point it seems to be something that genuine gets on her nerves, to the point I wonder if the first Lunar War wasn't to humble the youkai through a humiliating defeat.
I never thought about it like this, but after reading I actually totally agree 100%. While Yukari is a powerful character, her power isn't the reason she's Mary Sue-ish. It's the fact that:
She does whatever she wants and never gets punished for her dickery.
All her flaws are informed ones that never actually hinder her or cause any problems down the road (her laziness is never a problem when the story needs her to be active, for example). Contrast with Reimu, whose "slave to the PR" status and "act first think later" mentality both cause her trouble occasionally, or the Lunarians whose racism winds up getting their booze stolen and their purity obsession gets them driven out of their home by fairies.
Lastly, she ALWAYS wins in the end. No matter who she's up against she always defeats or outsmarts them. Her plans succeed 100% of the time, usually at little to no cost to Yukari.
She doesn't exactly have the "everyone loves her" status that most Mary Sues do, though she's not exactly detested by all either. Her relationship with Reimu is questionable, but they do team up often and Yukari even took Reimu away to train her. She's also pretty revered by Akyuu, who's arguably one of the most important people in the village.
So all in all, I actually agree that Yukari's the most Mary Sue-ish, even though she's certainly not the most powerful. At least the Lunarians get outsmarted and made fools of regularly, and Reimu is a super strong but still flawed protagonist.
Closest I can think of a touhou Mary-sue is Miko, but that's just because she tries selling other the image that she's perfect. Cue to all of the extravagant poses with sparkles and etc in HM. So in the end she doesn't even count.
But in a more serious note,
Wakasagihime is a mary sue because since everyone's body is 70% water she can grow in power infinitely!!!!