I think the purest, most classic narrative of the story is Rey’s descent to the dark side. She tries to take more and more responsibility on herself, and I got more and more frustrated (in a good way) with her failures the more she tried to take on herself. Her force lightning was such a powerful moment (despite the cheap reveal after). Then her darkest moment comes at the death star, where she throws Finn away and gets more and more angry at Ren, and as she goes in a downward spiral to the dark side she gets weaker. Ren is in total control besides a miraculous intervention with Leia, and Rey kills Ren at her darkest.
The best thing about is how those moments are all felt, not heard, and earned, not stolen, unlike most of the movie. That kind of characterization is what Star Wars is built on.
I agree. Arguably, of the three films where we see our protagonists unwittingly slip towards the dark (the third film in each trilogy), TROS handles it best.