One I personally really dislike, though many enjoy, is The Spider from imjusttryingtofindmyway.
Now, his fics are much better than the worst they have to offer. As a fic I once really liked, and the author of another fic I immensely enjoyed (Percy Jackson and the game if I remember the title correctly).
First, the main character is the best definition of a Gary Stu that I can at all think of. Dude possesses Peter Parker in a marvel world is the outset.
He is never a bad person, or isn't intended to (I view the desire to make a connected Hive Mind out of humanity as negative, but there's a debate to be had there). He is immensely OP, haivng the gigabrains that comics like to throw at people, which he used to gain the ability to integrate others' dna into his own. X-men and such being around, he uses it to get regeneration and other boons. But as soon as it starts getting out of hand (as power absorption tends to do), it's suddenly revealed that he is irradiating himself, sweating radioactive buckets at all times and slowly destroying himself from outside out.
Not too unreasonable - though he should be able to just make something to remove the radiation - but he reacts to this by deciding to not power steal anymore, and going to school while irradiated. He also learns how to use magic, and his death would cause the collapse of the multiverse, and a lot of other stuff that makes him the most unique, special individual in an infinite multiverse.
He also misuses his powers massively, forgetting and gaining them as the plot needs it.
Talking of school, his "father" orders him to go despite his insane smarts for social interaction. To the dude with a harem.
And yes, harem. At first he has a single chick he is into, there is some high drama for drama's sake, they break up and he ends up hooking up with a bunch of chicks, including invisible girl (him being a minor) and spider woman. As in the comics, he, spider woman and a bunch of spider-chicks are just attracted due to pheromones.
Besides him... nothing of note exists. Every character revolves around him and their interaction and disposition towards him. The world, besides being seemingly underpopulate (with the potential destruction of universes killing "millions", and crashing cities like NY killing tens of thousands), is unimportant and rarely develops in a way that doesn't directly tie in with him, too.
There's also dimensional travel, and his daughters from the future come back to talk with him (why is that a trope?) multiple times.
Not the worst fanfic by far, it's just the distiction between "hilariously bad" and "this had potential...", since the author is a decent write, and just doesn't seem to plan ahead at all.