As of my last update in September 2021, the following adult manhwa were highly regarded and popular among readers. Please note that the manhwa landscape is constantly changing, and new titles may have emerged since then. Be sure to check for more recent recommendations for the latest popular adult manhwa. Here are some highly regarded ones:

1. “Killing Stalking” by Koogi – A psychological thriller that follows the twisted relationship between a stalker and his victim.
2. “Sweet Guy” by Kim Carnby and Hwang Youngchan – A story about a man who gains supernatural abilities and explores his new life filled with romance and adult themes.
3. “Bastard” by Youngchan Hwang and Carnby Kim – A thriller that revolves around a troubled high school student and his serial killer father.
4. “Unholy Blood” by Lina Im – A fantasy manhwa with vampires, romance, and action.
5. “Seduce the Villain’s Father” by Yeonwoo – A woman transmigrates into a novel as the antagonist’s sister and decides to change her fate by seducing the villain’s father.
6. “H-Mate” by Turtle Feet – An adult manhwa focusing on romance and relationships.
7. “Household Affairs” by Seo Kouji – A story about a woman who rediscovers her sexuality after her husband’s infidelity.
8. “The Lady and Her Butler” by Jade – A romantic drama about a woman and her loyal butler.

Please be aware that adult manhwa often contains explicit content and mature themes. Make sure you are of the appropriate age and comfortable with the content before diving into these series.