I can't help thinking about this manga in 4 very distinct arcs of which, for the most part, I'd keep the first and obliterate the rest.
It's not fair to make that distinction, but up until Volume 14, Shamo had earned it's masterpiece status, at least in the world of martial arts manga.
It wasn't violence for violence sake, it was a study of a loathsome character, a true villain who, true to classic villain fates, had to fight odds stacked entirely against him but which he "deserved" due to his heinous, egotistical actions.

Story:
This follows Ryou Narushima, a brilliant teen who, out of the blue, knifed his parents to death, this being shunned and sent to a reformatory.
In there he was bullied, raped and, eventually taught karate by a shady yet talented figure. At this point, and arc is formed, where Ryou goes from desperate teen to victim of a harsh world, to the sprout of a villain that we will see grow in every sense all the way through to Volume 14, where the whole thing takes a dive into Crying Freeman, more or less, only to then turn and go back to it's grounded setting, but never recovering it's intense focus or purpose.

And that's the thing, there was a purpose, there was a line to follow, one which could end with Ryou's redemption or absolute fall into the darkness.
Neither came to be, though.

Characters:
Ryou Narushima is the absolute focus, the titular battle rooster with a deep feud against life. A man who ruined his own life when pressure got to him and never really looked back, yet not entirely devoid of ethics, morals or empathy. The tragedy of this is that Ryou isn't a psychopath, he's an insecure egomaniac, a survivalist, but also a thrill-seeker. A cunning mind, yet a thug. He lives by his own rules but he can't find happiness because he's unable to see himself for what he truly is. He can't ask forgiveness, he can't forgive himself and he can't escape his increasing number of evil deeds. So he's haunted by them.

Natsumi Narushima is Ryou's sister, who's life was essentially ruined by him, which she didn't take too kindly. She goes into her own spiral of doom, trying to survive, but finds herself perhaps even more lost than his sibling.
She's not properly developed and she's there mostly to be a burden over Ryou.

Kenji Kurokawa is Ryou's karate master, the man who changed his life by giving him the first tools to survive and shape his surroundings.
A mentor figure to him, he also carries a terrible evil within, some of which is passed down to Ryou in one way or another, being instrumental in shaping him as a selfish, violent man.

Naoto Sugawara is Ryou's first and foremost nemesis. The man who did everything right. A strict yet fair karate master who rose to the top by honing his talent and becoming an icon. Ryou hates him immediately.

The rest are other players in Ryou's story. If it isn't obvious yet, it's all about him, it is his character study fist and foremost, and all the other developments serve to move his story forward or add some nuance to it,
but these four are definitely the key players of first and better part.

Tragedy:
There was a point to all this. To all the violence and the suffering and the horrible things happening. This would either be a story about falling into the darkest pit and drown in his self-provoked misery... or a story of redemption from that darkest of pits. Unfortunately it was none.
Somewhere around the third arc, where another, more flashy nemesis was introduced to contrast and perhaps save Ryou from himself... the already unfocused authors had a falling out and got entangled in a legal battle from which, after a long time, the artist, Akio Tanaka emerged victorious with the rights to the manga. This was not so great.
While it's former right wasn't doing his best, at least he seemed to have remembered the point of it all and was moving forward in that direction.
Then the change happened and the point got further and further away.
The last 4 volumes of this are, essentially, painful to read.
Terrible characters are introduced, lots of threads are left loose and the conclusion to it all is an irritating, pseudo-poetic drag after the most ridiculous fight in the whole series. It's a mess.

So yeah, I'm giving the whole of Shamo a 7. The art reaches a peak and never goes down from it -some weird arm anatomy aside- while the highs of the story somewhat compensate the lows -although some of those basically destroy the initial point of it all. To me the first arc gets a 9, the third arc gets a 7, due to a lousy finish, and the second and last arc... they shouldn't exist. So a 4 or something.

Conclusion:
If you like martial arts stories, if you like dark stories, if you like to understand the origins of evil, to see the worse depths and not feel like you're reading adolescent edgy angst, there is a wonderful story for you... up to volume 14. Then there's a whole arc to skip until volume 17, where you get an interesting complimentary story... that falls flat at the very end, in Volume 28.
And then you evaluate your inertia. I can't deny there is no satisfactory conclusion, but sometimes it's about the journey, not the destination.
And skipping a couple of stations.