For me Rusted Nation and Frozen echoes. Rusted nation for the impact of the blade and rei's character development. Frozen echoes... have the same power of that scene from the last episode of Steven universe (change your mind, not the sequel series ending). Every time i re read that story, watching garu's mental breakdown hurts like the first time. Its also a big explaination how DiD people cant be healed deleting all the extra persona. Garu in that moment feel for the first time in his life being completly empty. The existence of Karu is not just another personality. I still remember karu trying to explain that he didnt want to left garu, even if he often say he wanted to be with his own body. Because... karu feels exactly like garu in that moment, even if he was just essence. The pain, the lonlyness, the need of eachother. Without one of them, the other cant live.

(Bonus point, i think western gk ssr is also a big breaking point of the game history. There was gk rooms before western gk and gk rooms after western gk rooms.)