Well what are you looking for in a "good" harem? I ask this because most JP/CN/KR harem novels are poorly written and manga/anime use them mostly for fanservice.
I'll just copy pasta myself : Regressor Instruction Manual. Overall fun story imo. Interesting protag trying to avoid dying while being chased around by yandere OP waifus in brutal non-Earth world with fellow Earthlings. Little Tyrant Doesn't Want to Meet with a Bad End. Heavily influenced by the Japanese harem novels, but does it better imo. Story's keeping me interested too in the overall plot while spending so much time bonding protag with the waifus. Not sure if it's going to end in harem, but it's a pretty haremesque development. Never Die Extra. This author doesn't like harems but his readers love his harem writing so much that he caters to them regardless. You'll probably want to check out his other works too since you want harems. This one's pretty standard type of story in a lot of ways, but that hasn't kept me away from it yet. A Demon Lord's Tale : DMGaHB (I'm not going to type that all out FFS). One of the few JP harem stories I've enjoyed (ever). Relationship development is pretty good imo. It's very Japanesy in a lot of ways, but it simply does it better so it doesn't make me want to drop the story. If every "laid back isekai harem" was written at this at this level, I'd be reading a lot more of them. I'm the Evil Lord of an Intergalactic Empire. I'm a bit hesitant to put this one here because for some reason it doesn't feel like a harem to me. It's a great story though and people classify it as harem, so I included it. Devil's Son in Law. Super harem. So much of the story is focused around it, and yet the action never stops. Only complaint I have is it seems like nothing happens (as in enemy characters don't do anything noteworthy) when the MC isn't close enough to deal with it, but overall is a fun read. And like other good harem novels (imo at least), great relationship development. Author even puts work into harem's inner relationship development (aka waifus spend time getting to know other waifus). Might be the closest to what you're looking for if you want some non JP harems. The Marquis' Eldest Son's Lascivious Story. It might be harem and badly written, but it makes no effort to pretend to be anything else. It's bad, you know it, but you're there for the action. Maybe I enjoyed this because protag isn't trying to bring his morals from modern day Japan to his new life.... he adapts instead of being your typical goody-two-shoes beta protag.
Date A Live - ln / Anime Highschool DxD - ln / Anime Mushoku Tensei - wn / ln / anime Monogatari Series - ln / anime Rokujouma no Shinryakusha!? - ln / anime (Although like only 1 season I think) Sevens (Never read it but it used to be really popular on NU) Monster Musume no Iru Nichijou The Misfit of Demon King Academy Sora no Otoshimono If you really wanna stretch the definition of harem, Index OT, NT and GT + Railgun - ln, anime for OT and Railgun (For love of everything that has ever existed and will exist, Read the ln over Index 3 at the very least. Railgun adaptation is nice so I would rec Railgun anime and Index LN) Baka to Test - novel, anime (it is just a love triangle tbh and same author as Grand Blue) Konosuba - novel, anime (I mean >.>) Oregairu - novel, anime There is also Haganai but I cant rec it considering the ending, I would say watch the anime if you want and pretend the series ends with the anime.