That's easy to say, because to an extent those are subjective things. I can say I hate Casablanca because of the poor acting, and you more or less have to take it at face value, but you can also offer a strong case in defense of the acting.
When I look at Star Wars and The Force Awakens side by side, I first see Mark Hamill / Luke Skywalker-- whiny, spotty acting ability, and a character who is virtually undeveloped. We have minimal backstory, and we see him wake up in the morning with no idea the Force exists, then blow up the Death Star before bed that evening. He receives nearly no training, doesn't practice, and doesn't develop or change as a person at all. But, the film was the first of its kind, it was a massive blockbuster, and it has been (completely justifiably) beloved by fans for 40+ years.
In Daisy Ridley / Rey, we have a much nuanced and developed character. Time is taken to show us her past, her fears, her motivations. We know she's spent nearly her entire lifetime alone, navigating harsh terrain, climbing through abandoned spaceships and fighting to protect herself. Moreover, when she finally has to stand and fight, she isn't as capable as Luke was. She doesn't "use the Force" to launch a perfect, direct hit on a billion-to-one shot to save the galaxy. Instead, she is nearly bested by her severely-wounded foe, before she is able to harness the Force long enough to push him away. The fight ends in a draw.
If you'd never heard of either film, and both were described to you without mention of gender, you'd select Luke as the Mary Sue, and Rey as the well-developed character.
So go ahead, argue whatever reason you want, but over and over all I see is whining about a non-existent Mary Sue, and an inability to accept a female in a lead role in a Star Wars film.
Luke is no Mary-Sue. The shot to blow up the Death Star wasn´t 1 in a billion since Luke was already confident when he said he used to bulls eye Womp Rats in his T16. Blowing up the Death Star was more about Luke´s piloting skills than his ability to use the force. Yes he may have used the force to help him target the opening but that´s it.
As far as Luke v Rey in the Mary-Sue standings. Luke gets defeated on several occasions across the OT. Firstly, by the wanted men in the Mos-Eisley Cantina, when Obiwan has to save his ass then by the remote on the Falcon. Darth Vader whoops his ass even after Luke has post Yoda training and is much more powerful. Luke could only start moving objects with the force, 3 years later after ANH, in Empire. Rey was already mind tricking Storm-troopers, moving objects with the force and besting Kylo Ren, a well-trained Jedi within days of learning about it. She actually disarms him before the ground separates them. Curious to know where Rey had "a draw".