ONLY COMPLETE SERIES:
Samurai Deeper Kyo: Shonen, Samurai,
NOTE: SDK is a Personal Favorite, but only for the characters... storyline is altogether too whacky for some peoples tastes. Put it this way... youll love it or hate it...
Rurouni Kenshin: Shonen, Action, Samurai, Historical
NOTE: Manga is more awesome than the anime... enough said... (if you havent seen the anime you should come out from under whatever rock you have been living under for the past few decades..)
Basilisk: Shonen, Action, Ninja, Historical, Tragedy
NOTE: Ninja warriors at their best... battles between warring clans leading to a bloody climax...
Get Backers: Shonen, Action, Supernatural
NOTE: Ban and Ginji are probably the coolest anime combo ever to hit the screen... and they're just as cool in print!
Kimagure Orange Road: Romance, Comedy, Supernatural,
NOTE: I hate the male leads guts... Ayukawa is mine I tell you!
Maison Ikkoku: Comedy, Romance
NOTE: Better in the anime form IMO, but the manga is good too.
Great Teacher Onizuka: Comedy, School Life, Slice of Life, Romance, Shonen
NOTE: Onizuka can be enjoyed in every format (Manga, anime and real life)... and never fails to please...
Hoshin Engi: Supernatural, Historical, Shonen, Drama, Comedy, Tragedy
NOTE: Dakki is probably the most evil Bia-tch I have ever seen in manga form... bar none... I think...
Monster: Shonen, Drama, Mystery
NOTE: If you havent heard of this gem by Urusawa... you should drop whatever you are doing and get it...
Death Note: Shonen, Drama, Mystery
NOTE: A must read... but for me the manga ended when a certain incident occurred. I only read the rest to get some closure. If youre like me... you will stop and pretend the manga ended when you reach that point.
Katsu, Cross Game, Hiatari Ryokou, Touch, H2, Niji Iro Togarashi, Miyuki, Itsumo Misora, Slow Step, Rough: Romance, Comedy, Sports
NOTE: Mitsuru Adachi is the God of sports rom-coms... his style is old fashioned, and you will think that some of his stories are similar to each other... but trust me.... much better than the inane crap you see out there nowadays... I am still not sure whether the Slow Step scans are complete or not actually...
MXO: Shonen, Ecchi, Comedy, Romance, Supernatural
NOTE: Cut brutally short for some reason... but probably for the best... its short, peppy and sweet...
Angel Denesetsu: Shonen, Comedy, Romance, High school gang wars (honestly this should be a genre by itself esp. for Manhwa)
NOTE: I already see a mention up there... looks like somebody has been busy... nice...
Ichigo 100%: Romance, Comedy, Harem
NOTE: Not a bad read... protagonist is the textbook indecisive guy... but the ending was refreshing in that the most obvious girl does not get the guy...
Slam Dunk: Sports, Romance, Comedy
NOTE: Probably my favorite sports comedy ever... no offense to Adachi... but the chemistry between the characters in this series is unparalleled... romance takes a back seat...
Nodame Cantabile: Music, Romance, Comedy
NOTE: Probably my favorite music based rom-com... ever... Nodames weirdness is unparalleled... romance almost takes a back seat...
Eyeshield 21: Sports, Comedy
NOTE: Probably the most profane manga I have ever read... (if the scanslators were translating properly)... the watered down anime does not do this series the justice it deserves... but it comes close I guess... as close as possible while omitting all the expletive's... which is half of Hirumas dialog!
Hikaru No Go: Sports, Comedy, Drama, Supernatural
NOTE: Not sure if we can call GO a sport... but whatever... pretty damned good read... esp if you enjoy GO...
Theres the other popular stuff I have been ignoring, but I will assume you know about them all... Bloody Monday, Parallel, Pretty Face, School Rumble, Inu Yasha, FMA, POT, Old Boy, Ranma, Gacha Gacha, Girls Bravo, Girls Saurus, Love Hina etc. Again... note I am only considering completed series here... Bloody Monday, Old Boy, Parallel and Pretty Face might be ones which you havent heard of though...
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Now that we have some of the basics under our belt... lets get off the beaten path for a bit
Show me the Money: Comedy, Romance, Drama
NOTE: I wish the story could have continued... but this manga is perfect as is... the premise is the most hilarious ever...
Meh... I am tired... more in a later edition...
[EDIT #01]
Anuki: Shonen, Manhwa, Airplanes, Drama
NOTE: It has airplanes and mercenary pilots... almost reminiscent of Area 88 but in a wilder past... art is grimy and scans quality (which I found) all sucked... but airplanes will always make me go on till the end... so... beware if you cant stand grimy scans and bad translations...
Furi Kuri: (Dunno what to say here.... really...)
NOTE: Its a blast as an anime... not bad as a manga either...
Mail: Supernatural, Mystery, Drama
NOTE: Another super short one... reminded me of Shinigami no Ballad in some ways... but a hidden gem...
Mercenary Pierre: Historical, European, Drama
NOTE: Its set in Europe during the 100 years war era... so yeah... lots of blood and guts... sexual content... pretty good for history buffs who dont mind seeing blood and gore...
Togari: Comedy, Supernatural
NOTE: It is pretty funny... reminded me of one other manga whose name I cant quite recall...
Living Game: Romance, Comedy, Seinen
NOTE: NOT the typical rom-com... and I do mean NOT... its got a whacky premise and it makes it work without stretching it too thin...
Spiral: Mystery, Shonen, Drama
NOTE: This is a pretty popular series... or so I used to think... but not many seem to have heard of it... its pretty good for mystery lovers... a bit like Conan in grown up form + some Higurashi types of characters...
Samurai Executioner, Lone Wolf and Cub, Vagabond, Path of the Assassin, Buddha, Bokko, Sangokushi: Historical, Drama, Samurai (not all)
NOTE: All are gritty historical dramas, the artistic styles are similar... and not the kind you see in regular manga... but all are fantastic... not sure if Sangokushi scans are complete though....