Sole Cloud Flying Cave
This definitely had one of the most interesting cultivation paths I've seen. The mystery behind how it all worked really drove the story forward. I do think everything after the original universe (and to some extent before that) suffered a lot from the crazy amount of power Cui Heng gained. He just completely outstripped everyone else and had no meaningful interactions with anyone. And, as for an overall flaw to the story, Cui Heng never faced anyone remotely close to his level of power. There was not a single time when he had to struggle. Heck, I don't think he ever even pulled out the Grotto Heaven, which was his absolute safe space even if someone was stronger than him. This led to all the fights being lacking in any real intensity.