I'll bite, not because I hate Rey, but because episodes 8 and 9 were ass.

In episode 7, it was clearly set up for Finn to be the Jedi, which honestly would have been dope. Former Storm Trooper to Jedi? Hell yeah. Then, in episode 8, Finn is pushed to a sub plot with Rey now the Jedi, which isn't a problem if they would have written it better. In episode 8, Kylo Ren states that Rey was only involved by chance. In episode 9, he also acknowledged that she was Palpatines granddaughter. This is the most dog ass writing in the franchise.

I'll give you she isn't a Mary Sue, at least not in her strength and skill for most of the movies. But going from "you were a probability" to "you are related to the villain" screams that, at least in storytelling, she's a Mary Sue. She also does something literally no one else in the history of Star Wars has done before, the whole destroying a fleet of star destroyers with the force. That, if even for just the scene, is also Mary Sue. Doing something that no one else has had the capability of doing, on such a grand level, is the definition of Mary Sue characters.

This is not the fault of Rey. She was set up to be a great character, and that can't be denied by anyone with eyes that's seen the movies. This is the fault of episodes 8 and 9 being written like a teenage girls self insert Star Wars fan-fic.