(Some of those have only 9 volumes released but I hope one volume doesn't make quite a difference)
Still nobody said
Vinland Saga? I mean, I can't really be the first to recommend it...
Claymore same as above.
Angel Densetsu same mangaka of Claymore, personally I found this a lot better tahn Claymore.
Historie is only 6 volumes so far, but since it's the best seinen going on imho I've to suggest it.
Gunslinger Girl is a shounen, but it's more similar to other seinen than other shounen (it's slower in term of pace than other seinen, so if you prefer straight-forward action I'd try something else before, but this is the best manga ever imho so I think it worth at least a try). It has by far the best character development I've ever found in anything.
Monster and anything else by the pen of
Urasawa Naoki.
Dance in the Vampire Bund I'm not very update on this, but I enjoyed the start.
Kurozuka I think this one is pretty underrated. I liked it a lot.
and when you're tired of action:
Yotsubato! the king of the refreshing manga (same thing as Gunslinger Girl, it was on a seinen magazine who became a shounen magazine but its target are people older than teens bla bla bla)!
Genshiken well it's Genshiken, a must if you like manga.
Kodomo no Jikan you said you didn't care particularly about genre right? Then well, I personally think that behind the facade (and I know for some people it's hard to watch behind it in a manga like this) there is a very good manga. There is no more the lolicon tag, but if you have problems about pre-adolescent girls talking about sex you shouldn't even start it.
Manhwa:
give it a try to
Ares (shounen). Art is pretty strange many people really dislike it (I think it was fine honestly).
Unbalance x 2 my favorite romance seinen manhwa (not that I read many of them lol). It went recently running again from hiatus.
The Breaker a must (seconded).
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What I like in Trivial Pursuit style. Pick your category:
