Around season 5 of Buffy, there was another perfect Mary Sue, only, unlike Jonathan in season 4, she wasn't played up as a joke. A former boyfriend, Riley Finn, shows up in town, and his new wife, Sam Finn, is a textbook case.
Her backstory is that she was some sort of volunteer nurse in a third-world country, only a few months ago, when her team of charitable do-gooders were wiped out by demons. Despite this Mother Theresa-esque backstory, she's introduced to the narrative swinging down on a bat-line to kick the butt of a demon that is handily throwing Buffy (who has actual super-powers, and has spent five years fighting these things) around. She's better looking, taller, smarter, a former social worker / doctor, married to Buffy's ex-boyfriend (who is implied to have been psychologically damaged by his relationship with the self-absorbed Buffy, and only Sam 'saved him'), etc.
She gives magical advice to Willow, who has spent years becoming the most powerful witch on the planet, and has an advisor who has access to the best magical resources on the planet and has been dabbling in magic since the days of Woodstock. But Sam Finn, former social worker, knows better than all of them!
She gives relationship advice to Xander and Anya, whom she's just met and knows nothing about. But Sam Finn has the magical words of wisdom pulled straight from her perky derriere to make things right!
She offers psych and fighting advice to Buffy, the 'one girl in all the world, Chosen to fight the monsters.' But Sam Finn, some random bimbo who just discovered the world of the supernatural, and has no super-powers at all, is conveniently better at this, too, just as she's just automagically better at everything else.
She's the perfect Mary-Sue, being better at any single skill than over-specialized masters of those skills, *and* being presented as morally virtuous, and placed in a position to *lecture the hero* when Buffy is set up to fail at a task.
From the moment she bat-lined into the scene, to her final words of wisdom on the way out, her entire presence one-upped everyone else in the show and made them look inferior or damaged or weak in some way.
(And that's the worst kind of Mary-Sue, one that isn't just better at everything than everyone else, but someone whose presence inexplicably makes everyone else dumber, weaker and less competent than they were before she arrived.)