I'd recommend 'Hidden Hearts'. It's underrated perhaps because it doesn't follow the typical yaoi tropes. The characters have complex emotions and the plot has unexpected twists.
Maybe it's due to lack of promotion or not fitting the mainstream taste. Some underrated hentai manga just don't get the attention they deserve.
Maybe they have a niche theme or style that doesn't appeal to a wide audience. Or perhaps they lack proper marketing and promotion.
There could be several reasons. Poor marketing can make them less known. Also, if the themes are too niche or the characters aren't engaging enough, people might ignore them.
Often, underrated manga suffer from poor marketing. They might also be in a niche genre that doesn't get much attention. Another reason could be that they were released at a time when there was too much competition.
I'd recommend 'One-Punch Man'. It's got great art and a unique story, but it doesn't get as much hype as it deserves.
I recommend 'Bakuman'. It's about two teenagers trying to become manga artists. It has a unique plot and great art. The underrating might be because it's not your typical shonen manga, but it's definitely worth a read.
One underrated romance manga is 'Kimi ni Todoke'. It has beautiful art and a touching story, but maybe not as popular as some others. The characters' emotions are depicted so realistically.
You could try 'Saezuru Tori wa Habatakanai'. It shares some themes and intense character dynamics similar to Hyperventilation yaoi manga. The art style is also quite captivating.
You can start by promoting it on social media to reach a wider audience. Sharing positive reviews and recommendations can help.
You should check out 'Enchanted Sparkles'. It has great world-building and relatable characters. Maybe it was released at a time when there was a lot of competition in the genre, so it got overlooked.