Looking for recommendations! My personal pick would be Blooming Love (MangaPlus) for being a very wholesome and somewhat realistic romance manga featuring actual adults.
Here's a list of the stuff I tend to link from time to time. They're not perfect and vary on how underrated they are but all of them managed to stay with me in one way or another so that's something.
Sui and Neri of the Twilight Planet - a cute (and short) slice of life series about a girl and a talking sloth running an inn on a dying Earth. Art reminds me of Girls' Last Tour (also read that if haven't and want some post-post-post apocalyptic adventures).
Satellite Coin Laundry - sci-fi comedy about a space laundromat. It has some great panels as well as interesting aliens and ideas. Translation doesn't update particularly often unfortunately.
Traveling Gyaru - fun travel story about a gyaru who decided to bike through a good chunk of Japan for her friend's wedding while knowing nothing about traveling on a bike. Nice art, interesting trivia about traveling and locations but not fully translated (last mangadex updated was over 2 years ago).
I also like your flaws - an awkward boy and an amputee girl meet during their first day at school, start hanging out. It's a oneshot and could use a serialization to expand on the characters but it's a neat read.
Pen Pal at the End of the World - another oneshot, this time about a young girl and a former artist exchanging letters in a post-apocalyptic setting.
The Unwomanly Face of War - World War II stories from a perspective of women who served in the soviet army. A unique look at a "overplayed" topic. Based on a book with the same title.
Kowloon Generic Romance - romance, drama, sci-fi? It's a slow burn series about life in modern day Kowloon. It has both goofy and striking panels, some a bit more risque but don't expect a lot of fan service. It also has an official English release.
I don't really have a great sense of if something is underrated or not, but here are some great series that I don't see mentioned a lot here:
Sweat and Soap - This might not qualify as underrated, but it is a fantastic love story between adults
Love After World Domination - This one was cancelled after the anime didn't drive sales, but I absolutely loved this series and would revive it if I could
Love's In Sight - A fantastic story about two outcasts from normal society falling for each other.
A Story About a Droid - This one was dropped by the author (it was a Twitter series), but I really enjoyed what exists.
Ubunchu - An old series that was published in the Ubuntu magazine in Japan. I find it hilarious, but you probably need to fit my exact demographic profile and linux experience for it to hit as hard.
Slime 300 - I am not fully up to date on this series, but it is a great, slow-life isekai. There are never any stakes and is just a comfy time. Season 2 of the anime is coming this fall iirc.
I Decided to Fake a Marriage with My Junior to Shut My Parents Up
I have this one bookmarked to come back and read when I get a chance. Looking forward to it!
Naked Hero
I read the first couple chapters a while back and it was pretty funny, but dropped it for time. I might have to pick this one back up after I get back from vacation.
The Voynich Hotel is a wonderful manga that i never see talked about. An ex-yakuza goes to a small island to escape his past and stays in the titular hotel. It's a slice of life manga in a world that is very weird.
I'll take this to mean both underrated (I gave it a rating decently higher than mean/median) and also ones that are not popular.
- Champagne If you've ever thought of reading Bartender but want to try something shorter, this is a good introduction. The ending, while a reasonably common trope, still kind of surprised me.
- Kako and the False Detective Another short read that sticks the landing. I don't know much about mystery/supernatural manga, even including body horror, which this isn't, but it was enjoyable. I liked Kako's character and design, and the plot is good.
- Sleepy Barmaid (ongoing) One of my true favorites. MC lives to the beat of her own drum.
It's a comedy/slice of life (imo) with horror aspects. A girl is suddenly able to see spirits/ghosts one day, and tries her best to pretend she doesn't see them. I love it because each arc and Mieruko's intentions are so wholesome & end in building way more on spirits' rules and lore. The first few chapters do play on echi themes, but the author finds his footing for a plot once the first antagonist appears. Antagonist is not the right word, but I don't want to spoil character development details much. :x