In terms of plot, this manga is my least favourite out of the Naoki Urasawa power trio of Monster, 20 Seiki Shounen and Pluto. I know that view is in direct antithesis to majority opinion, as this is unarguably the most popular out of the three in both Japan and on this site, but Monster is basically about a bunch of people who don't have the guts to empty a clip or two into the brain of a guy who goes around killing people (most of whom are good people) just 'cos he feels like it. And their indecision and misplaced sympathies destroy about 50 more lives into the bargain, despite there being no earthly reason for any of them to hesitate in killing him: he ruined all of their lives and killed most of their loved ones. By all rights, none of them should have a problem finishing him off, especially as he stands unarmed in front of them while they have loaded weapons. In Pluto, no one knew who the villain was; in 20 Seiki Shounen, they knew, but could never get close enough to him to do what had to be done. In Monster? Even at the very end, there was nothing but hesitation in the main characters' actions when facing down Johan.
Johan is the next big issue. He doesn't want anything: not power, not money/possessions, not popularity/recognition, not influence. He's not a sadist who is aroused or excited by killing. He's not an adrenaline junkie. He's not delusional. He didn't suffer any particular trauma in his childhood: it was a difficult childhood, yes, but not a hugely traumatic one. Why on earth does he murder so many people? It's not desire/lust, not megalomania, not greed, not insanity, not survival instinct, not vengefulness, not...anything. He is nothing. I know this was intentional, was meant to drive home the point that he was The Monster, but, to me, he just ended up being an annoyance. I seriously doubt there's a single person in this world, serial murderer or no, who kills for absolutely no reason. (Maybe it's just weird to me because I'm not Christian/Catholic and therefore don't believe in absolute evil. Many people are born evil, yes, but even evil people have various reasons for killing. If nothing else, they don't do it willy-nilly because murder's too much of a hassle to go through unless you get something big out of it, while Johan has trivial characters who barely even knew his name/looks killed off because of heaven-only-knows what)
That being said, the mangaka is Naoki Urasawa, so... The plot twists are brilliant. The story-telling is stellar. The flashbacks are perfectly timed and skilfully portrayed. The emotions are incredibly moving, which is a real compliment coming from me because I find most emotional scenes in fiction to be ludicrous. The art is fitting, and everyone looks so unique that despite the huge cast, it's nigh impossible to confuse one character with another. The people (save for Johan) are actual people with a diverse variety of pasts, backgrounds, dreams and hopes -- not cardboard cut-outs who only mirror the author's directions. Tl;dr -- Monster incorporates elements many thrillers fail at and does them justice -- hence my high rating.
If only Johan had been more well-rounded as a villain (apart from "evil genius," he was a total blank) and the circumstances of his continued existence less absurd (it's basically: "The MC's got cold feet so the Big Bad escaped...again") Monster would have got 10/10 from me.