As usual, there may be some minor spoilers below, but I won't spoil anything substantial.
Monster is the first anime that I have encountered (I'm still a little new to the genre) with the gritty realism to match a standard television series - which is probably why there are currently rumblings about it being turned into a live-action HBO series. Most anime have a strong fantasy/sci-fi/supernatural element, but Monster is very much set in the real world, following real world parameters, and surrounded by the darkness and consequences of real world actions.
STORY - 9.5/10
Dr. Kenzo Tenma is a brilliant surgeon who is faced with a difficult decision early in the series - follow his boss' orders, or perform a surgery that only he can perform to save the life of a young boy. He chooses the latter, and sets in motion a string of events that would have been impossible for him to foresee. And yet he spends the remainder of the series wracked with guilt over that decision, and doing everything within his power to track down the boy - who is now a young man - and stop the reign of terror that the boy has unleashed.
Pretty much the entire story revolves around the boy, Johan. Johan is the titular monster, and the story slowly peels back the layers revealing, little by little, just how Johan became the monster that he is. I got a very distinct Harry Potter and the Half-Blood Prince vibe from the way that the other characters, predonimnantly Tenma, learn of Johan's past - just as Harry slowly learns of Voldemort's history. The idea that understanding how someone came to be who they are might be the only way to stop them is an interesting one, and the show spends a lot of time trying to figure out just who Johan and his twin sister, Nina, are.
More interesting, perhaps, is how Monster chooses to tell its story. Short arcs will unravel, and just when we think that we're about to find out something pivotal, the plot will back off and introduce completely new characters and begin to tell their stories - eventually leading to the place we thought that we were initially going, just from a different angle. While this can be frustrating initially, once you learn to trust that the writer knows what he's doing, it's actually an engrossing way to let the tale unfold. I found that it added layers of complexity to the plot that would be simply unachievable otherwise. Some might complain that it adds too many side plots and unimportant minor characters, but I would argue that those people are missing the point of the story altogether.
There is a constant gloominess surrounding the plot, which can be attributed to Johan's almost god-like powers to control everything that is going on around him. The story also touches on a lot of uncomfortable and dark ideas (mostly involving the treatment of children). This causes much of the story to be grim and depressing, but Monster does an excellent job, usually through Tenma or Nina or Dieter, of demonstrating glimmers of light and hope amid the darkness. And, without giving too much away, the story does conclude on a happier note.
ANIMATION - 8/10
Monster isn't as visually impressive as a lot of more recent anime. It lacks the dazzle and flash of series with lots of action, and the grittiness of the animation almost makes it feel like it's an entirely different genre of series altogether. But it works within the context of the world that the show is trying to create.
My main pet peeve is that the animation of actions is often clunky (people running, in particular, just looks really off in this series). But on the whole, the look of the show definitely adds to the ominous vibe that pervades every frame - the hopelessness that anything can be done to stop Johan.
SOUND - 8/10
As usual, I watched the English dub of Monster. I found some of the voice acting to be outstanding, while a number of the voices seemed somewhat jarring - they just didn't seem like they would belong to the character on the screen. But the main characters were all well done - Johan, in particular, has a haunting, yet soothing, quality to his voice that is really unnerving.
The series' score is also strong, and I often found myself entranced by the music during important scenes.
CHARACTERS - 10/10
Like any great series, the true strength of Monster is its characters. Dr. Tenma is an appropriately broken man through much of the story, unsure of his place in the world after the decision that he made. Even the look of his character shows the weight that he carries around on his shoulders. But despite all of his doubt and guilt, he always tries to do the right thing - the very quality that caused him to save Johan in the first place. Tenma is a simple character in that regard - you always know that he will do the right thing. But there is still a complexity to him that is intriguing, and he is an easily sympathetic character that all viewers will identify with and cheer for.
Johan, of course, is far more intriguing, as most well-constructed villians will be. Johan is shown as the embodiment of evil, the cause of countless deaths across Germany, and even stretching into Czechoslovakia. But the truly terrifying aspect to Johan is that he's rarely the one who pulls the trigger - he has the ability to compel others to do his bidding. He is chaismatic, and if we weren't privy to the fact that he is who he is, he might come off as a very likable character. In fact, as we unravel the history of Johan and Nina's past, we might find ourselves feeling a certain amount of sympathy for Johan. Nina (an overly good character, despite her own troubled childhood) serves as an example of what kind of person Johan might easily have become, had certain things never happened to him as a child.
There are a lot of secondary characters in Monster. Some of them are important, and others less so. But almost every one of them is given personality and nuance, and there are very few series (anime or otherwise) that achieve as much with as many characters as Monster does. Many of the key characters show a tremendous amount of development over the 74 episode span of the series.
The series tries to make the emphatic point that every person has a monster inside of them, but that each individual's choices determine whether they are good or evil. The constrast of two characters like Grimmer and Roberto shows just how easily a few choices can lead people with very similar backgrounds down entirely different paths. Eva, Tenma's ex-fiance who can't seem to come to grips with the fact that she still loves him and spends a decade trying to replace him, is another excellent example of a complex character who shows excellent growth over the length of the series.
All in all, I would put the character work in Monster right up alongside a series like The Wire, I was that impressed with it. I haven't seen anything quite so involving in an anime yet.
OVERALL - 9.9/10
I have a difficult time giving out perfect scores, but Monster is as close to deserving of one as I've seen in an anime series yet.
The story is incredibly well-crafted, and the unique way that it unfolds adds so much complexity to the world that Monster builds. The characters are all well done - even the obvious heroes have flaws, and the obvious villians garner sympathy (except maybe Roberto). And there are plenty of characters who are more ambiguous. And the series touches on plenty of weighty themes and ideas (more than I could ever get into in a review - you could easily write an academic paper on this series).
I would highly recommend this series to anyone who enjoys a more serious and deep story.