This anime has a quite boring beginning (first couple of episodes), but as it unfolds the main-plot, it becomes one of the greatest fighting animes I have seen and it doesn't drag out, but is focused on telling a story. The story that is told is great, maybe the greatest story I have seen in an anime. That is why I would rank it in my top 3, already. It is not only the story, but how the story is conveyed that makes this anime so great.
The plot focuses on Tatsumi who is a boy from a poor village travelling to the capital to earn some money to save his home-town. He has learned to fight as a kid, being taught be a teacher in his village. He then meets a band of assassins and the story unfolds. I won't spoil any of the action as everything is part of the build-up, which makes this such a great anime. It is very simple, too. This makes it even more impressive, because I am quite shocked that no-body had made such a story-line before (in such a context).
The theme of the series is assassination, heroism and social values (good and evil especially and love of course). It is the value-part and how it is conveyed that makes this anime into something special.
The animation is quite great. It is simple, yet characteristic, and the fights are really great.
The main focus is the progression of the story, though, and how the characters are related to in the sense of the main theme, which is good and evil I guess.
I give it a 10/10, because it surprised me and tried something new and I liked it all together - better than most shows. I would compare it to "Attack on Titan", which is also a new show. AOT focuses on mystery and survival, while AGK focuses on societal and heroic problems (almost solely). I liked AGK better, even though AOT has better animations, because of the message AGK has speaks to me more! It is simpler, yet more striking.