Technically, this is urban fantasy rather than classic isekai, but Issei Hyodo is reincarnated as a devil, which feels like a different world. This is the horniest mainstream anime ever created, but it has an unironically amazing power system and war drama.
If there is a modern bible for Isekai Harem Monogatari, it is Mushoku Tensei. Often credited with popularizing many tropes we see today, this series follows a 34-year-old shut-in who dies and is reborn as Rudeus Greyrat in a world of swords and sorcery. isekai harem monogatari top
Rio is a reincarnated orphan who discovers he has the memories of a Japanese university student plus the magical heritage of a lost spirit people. This is a slower, more melancholic isekai. Technically, this is urban fantasy rather than classic