I'm not much into fanfics, since I consider those mostly practice for original creating writing than anything else. That's not to say there can't be good ones, though. This one I will talk about simply had no right to be as good as it was.
It's a Sailor Moon fic called "I'm Here to Help", about an insane guy from the future who goes back in time in an attempt to undo it, claiming it's a total dystopia that suppressed humanity's free will. For those unfamiliar, the second season of the anime states that Sailor Moon is set to be elevated as Queen of the World and usher a peaceful post-scarcity utopia where every single person would get to live and keep their youth for thousands of years.
Two kickers: One, he's so bugfuck insane and lived for so long prior to his travel, he literally forgot who he truly was and is only doing this quest of his out of some twisted sense of duty to all of humanity. Two, he's actually right and suffered Sailor Moon's wrath first-hand for daring to combat her incredibly skewered idea of a perfect society.
It's incredibly well-written, with the main character's thoughts and motivations being shown in a series of disjointed notes found around the world and spliced between scenes of him trying to stop the Senshi in his own way, making him teeter between a byronic hero and a tragic anti-villain. Not to mention it questions the heroines' true motivations and whether it is for the better of everyone that things should go like that.
