I think a lot of folks default to 'A Court of Thorns and Roses' when they want supernatural RH, and that's fair, but I'm kind of tired of how central romance has become in that genre. My vote goes to the less obvious pick: the 'Zodiac Academy' series. Yeah, it's bullying romance and gets super messy, but the supernatural system built around star signs and elemental fae magic feels more intricate than most. The harem aspect develops slowly across a ton of books, which can be exhausting but also gives it a sprawling, soap-opera feel. It's not high literature, but for pure addictive plotting and magical school politics, it's weirdly effective. I got my sister hooked on it last summer and she blew through eight books in a month.
Another one that doesn't get mentioned enough is the 'Dark World' series by Angel Lawson. It's urban fantasy with vampires, werewolves, witches, the whole crew, and the reverse harem is pretty central from the get-go. The writing can be uneven, but the dynamics between the different supernatural types in the harem are what kept me reading. It has that 'found family' vibe mixed with constant danger, which hits a sweet spot for me.