I'd argue against the Watatsuki's being called Mary Sues (because really, they're not), but no-one would listen to me
Even if they're not the perfect description of a Mary Sue, they pretty much were just written as characters who could pretty much beat anyone by barely doing anything. Even at the end, they were never really beaten per se, they just got tricked. Even Yukari has a bit of reason for generally being able to obliterate everything, but even she can lose disputes through danmaku (coincidentally, it was implemented for cases like this and is likely as such the only way for her to lose a fight).
Of course, the only reason that they were so much stronger than anyone was the lack of spell card rules to follow and other such devices, but either way it's easy to see how everyone describes them as Mary Sues. I myself think the same. Of course you can have a villain in a story that originally beats the protagonists because that's just a general plot point, but it really was a bit off.
This.
If it was
me doing it, I'd've (a) gone out of my way to show the protagonists' strengths, and (b) shown Yorihime with
skill, not hax. She'd win, not because she goes "Can't touch this!", but because she's an
equal or better fighter. She'd lose to Reimu, not because the latter goes "no u," but at least in part because she was getting worn out. (For maximum comedy, she'd be so worn out that Reimu would KO her by flicking her forehead. And then she'd carry her unconscious form to the Capital.) And Toyohime's "destroy-everything haxweapon" wouldn't be just a fan. It'd be something physically
huge, and we'd see it beforehand, with hints as to its purpose.
I still can't think of anything that could possibly top
, though.