Alright, so I've read a ton of these, and honestly most of them are bad. Not just bad, but boringly, predictably bad. The whole 'Ash gets all the girls' premise is fun, but it gets old fast when every story is just a checklist of female characters blushing and fawning. The thing is, 'harem' doesn't have to mean shallow. I think the compelling ones treat it like a character study.
Forget just throwing Misty, May, Dawn, Serena, and every gym leader into a room. Pick a dynamic and explore it. What if Ash, after years of traveling, finally gets a clue? How does that change his interactions? Does he become more confident, or is he awkwardly trying to navigate it? The girls shouldn't just be trophies either. Give them agency, jealousy, maybe even form alliances against him or each other. A story where Iris and Misty constantly bicker but team up to prank him is way more interesting than them just sighing in unison.
Also, lean into the Pokémon world. The journey is key. A harem fic that's just a series of dates in generic locations misses the point. Make the romantic tension happen during battles, while camping, dealing with a Team Rocket plot. The shared adventure is the glue. And for the love of Arceus, give Ash some actual character progression beyond being dense. Maybe winning the Alola League changes how he sees himself and his relationships. The best fics I've saved use the harem trope as a vehicle to actually develop everyone, not just pile on characters.