Is this only for manga magazines or all genres? If all, I love Akira, god of all classics.
Whatever type of manga you like, go for it.
Mahjong Legend Akagi, although the anime they made based on it is even better. Yes, it's about a guy who plays mahjong, and it might be the best thing ever.
That sounds quite interesting, who writes/illustrates it?
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These are the titles I follow/ed and own, excluding 1-shots and the ones I wasn't really into:
On-going:
Bizenghast
Kurogane
Loveless
Someday's Dreamers: Spellbound
The Tarot Cafe
Translucent
xxxHolic
Yotsuba&!
Complete:
.Hack//Legend of the Twilight (3 volumes)
DeathNote (12 volumes)
Kamichama Karin (7 volumes)
Rurouni Kenshin (28 volumes)
Someday's Dreamers (2 volumes)
I have a fairly decent collection of seinen manga, and a fair smattering of shonen. Lately I've been getting more into shojo manga for some reason, despite having gotten to the age where a lady normally starts growing
out of that kind of thing. The Tarot Cafe is really the only josei series I've read, which is obviously odd as I'm exactly the demographic they're targeted towards.
I've looked up a couple other series intended for the adult female audience lately, so that might change; shojo manga, especially romance, tends to be too idealized and is thus highly unrealistic (not to mention silly). They're fun for those moments when you want to quit thinking for a while, but josei is probably more my speed these days anyway.
Why I enjoy seinen so much, though, I have no clue. Series marketed for more mature men, like Yotsuba&! and xxxHolic just appeal to me somehow in their humor, mix of realism/surreal themes, and lack of boobs all over the place (a great improvement over most shonen).