Anakin Skywalker was the Chosen One with a higher Midi-Chlorian count than Yoda. After 13 years of training he was still weaker than Yoda.
Luka Skywalker was the son of the Chosen One - after 4 years of training he was still weaker than the Emperor.
Ben Solo was the grandson of the Chosen One - after two decades of training with Luke Skywalker and Snoke, he was still weaker than Snoke who in turn was weaker than Palpatine.
Rey Palpatine was the granddaughter of Palpatine. Some 2 days after she found out that the Force existed, she managed to best a wounded Ben Solo. A week later, after receiving 2 days of "training" from Luke Skywalker, she was equal or superior to a healed ben Solo. One year later, she was equal or stronger than Palpatine.
Rey is a Mary Sue. Ben isnt a Gary Stu because he had - and needed - training to get to where he was.