Which one? I think they're all good, but
the "7th gym". We don't get a town because of it and the "gym" sucks as it's just a linear pathway. 8th gym has a great idea with the weather focus and the trainers in it using these moves, but the fact it's just a room with 3 NPCs makes it kinda ridiculous as it was clearly unfinished. It needed a "dungeon" based around weather changing, would be fantastic.
it was raihan's actually. just three regular battles? are you kidding me? couldnt you at least find a gimmick to use like kabus gym did so well? i dont even consider that stretch of walking for gym 7 as a mission, its pathetic. though i did like the fight with piers and no dynamaxing