Been reading it lately, and it helps reduce my scrolling time. I've hardly read any, so you can recommend really popular stuff, too.
I've read Vagabond, 20th Century Boys, Claymore (years ago), and some berserk. I just finished reading Teppu, which I thought was an interesting subversion of a lot of anime tropes. I also liked that it was a short run (only 8 volumes). I guess I like seinen, but I've also enjoyed josei like She Loves to Cook, and She Loves to Eat.
Anyway, no shonen please. Hard mode: please nothing about high school
"Gon" by Masashi Tanaka. No words, just a story using images. You don't have to read them in any order. It's about a dinosaur who survived to the modern day and just vibes around other wildlife. One of the best drawn comics of all time.
I've been wanting an English translation of Shigurui: Death Frenzy, but no official one exists. If you can read Japanese or don't mind fan translations, you'll like it if you liked Berserk and Vagabond. It's about two rival samurai (one blind and the other with only one arm), how they became rivals, how they got their injuries, and how they finally have a chance to kill one another when a mad lord holds a tournament where people fight to the death.
It completely strips away the romantic version of samurai, while showing how awful the caste system in Japan was.
I actually watched the anime and it was really cool. Does the manga continue the story, or does the anime cover it all?
AFAIK the manga continues for a bit after the anime. They were going to make season 2 when the author finished more, but then the author never finished the series (or it's still ongoing).