In my top 3 Romance Fantasy series, I've been sold on this series since chapter 1. It's long and has both funny and dark elements (the comments say it suddenly got dark but it starts with a massacre so idk why they think so). It's got great characters and a plot that, while long, makes sense when it comes to the overall vision. It took a solig 16-ish years to get to what I believe to be the final arc, but I have not skipped a release and suffered when the scans were on hold for a
*Novel-Style Isekai
Superior to the original Fushigi Yuugi (this is the prequel), this is a well-rounded story with a great FL. This series is a tragedy, like I cannot stress that enough. There is a sort of bitter-sweet happiness to it, though. The events are more or less summed up in the original Fushigi Yuugi because it serves as a precedent for the future events. It's dear to my heart because I followed it since it started.
Contains: Otome-Style Isekai
This is among the best of the best when it comes to the “villainess from an otome game” trope.
Contains: Novel-Style Isekai, FL becomes the protag as herself. It may not be the greatest but it's a verified classic in the genre. The conflict between best friends is one of it's better selling points. I would however recommend Fushigi Yuugi Genbu Kaiden (a prequel with the same concept but further back in real-world history) because it's better written and more emotionally impactful as the author wrote it much later.
Isekai series where MC falls into another world. It's interesting in that she actually has to learn the language and customs rather than somehow knowing them immediately.
One of the funnest villainess series of the bunch and a personal favourite of mine. Enough to have read multiple times. Male lead is her assistant (henchman) and they have fantastic chemistry. She only acts the part of the villain but is rather tsundere about the whole thing.
Akatsuki no Yona