Here is my take on it, nothing too brutal or dark but more psychological:
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Holy land https://mangadex.org/title/916 by Mori Kouji.
Very good fighting manga, more realistic than any battle manga I have read but the main selling point is it's deep take on the psychology of the delinquent on the street, and the development of all the characters in the story, not just the main protagonist. Super recommend!
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Arigatou https://mangadex.org/title/6077
Weird(rape, excessive violence, but the overall mood when reading is just weird) first chapter, actually the weird mood continue through out the whole story which, helps the readers dealing with the depressive events happening in the story. It's main theme is about being optimistic in hard times and hope.
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Ressentiment https://mangadex.org/title/1773
Another story about VR girlfriend? Perhaps, but this one take a more realistic approach to the theme, dealing with the main character(30 y.o) psychology. Nice story if you want to hypothetically peek into what will happen if the VR technology proceed to far.
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Claymore https://mangadex.org/title/303
I know, the tag is shounen but just read it mate. I mean it. Good fight scene, super nice monster art, emotional and much truer take into how far your effort can take you without natural born superiority.
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Franken Fran https://mangadex.org/title/623/franken-fran
It's fun, makes you wanna puke and barf at times, simply disgust at others and intrigue you most of the time but overall, it's a very entertaining read. (It's a light-themed horror that is more about making you feel strange and disgust, not much scariness here).
Here are some more light - hearted seinen, hit or miss depending on your choice but all are very good manga:
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Himegoto - Uniforms at the Age of Nineteen https://mangadex.org/title/6925
Nice romance manga. Think of it as Good Ending (GE) with a more likeable(imo) cast and better psychology. The story is about somehow twisted love between a crossdresser guy, a girl who cannot let go of the past and another girl who is broken inside. Same author as First love zombie(damn, Shounen sunday serialization really change him, then again, maybe not).
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High Score Girl https://mangadex.org/title/9530
Super cool and cute story about a boy and girl who loves arcade games that live in the 90s. The retro arcade games of the past fit really well into this story of growth and slow realization of love between both of the main character.
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Gokusen https://mangadex.org/title/1154
A bit far-fetched as this is a Josei but the fact that this manga is so damn hilarious and entertaining makes me wanna recommend it to you so bad. The story is about Kumiko - a fresh out of college teacher that come to teach at a delinquent school. Her secret is that she is also the heir of a big yakuza clan.
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Kuro https://mangadex.org/title/11140/kuro
A very cute suspense story about a girl with her cat(?) that live in a town plagued by monster. Come for the cuteness, stay for the suspense, but don't take it too seriously.
And, that's it! Hope my list help you find some thing you'd like!