I know in a lot of reviews, it tells you practically everything that happens. I will try my best to avoid that. You can read a series summary on this site for it. And any summary really doesn't due this manga justice. Instead, I'll tell you why I gave each category its rating.

Story=9
This story goes much more in depth than the anime makes people believe. There is action, adventure, sadness, humor, suspense, and all the good things that make a plot line great. But you might ask why I didn't give it a ten then.
Here we go; there is suggested romance, but it just isn't enough. I know this is a shonen, but I have a little pet peeve about it. There was just enough romance that you know there is something there. However, that little peek is more annoying than anything else. There should have been just a little more.
That wasn't my only reason though. It went from having this one bad guy, who was made to be evil incarnate, to a completely different bad guy. It wouldn't have been so bad if the transition was slower, but it happened pretty abruptly.
There are also a couple confusing occurences that happen. Only the most observant readers will notice them (me included). If you do,none of them affect the plot or really stand out, but it just makes you wonder. (You'll know what I'm taking about when it coems to the Hair thing with Kyo when he was a child) So I can't say the story is perfect.

Art=10
The drawing style was amazing, to me. It stayed the same throughout the whole series (which is a good thing), and the characters didn't suddenly wear something completely different or have a miraculous scar that wasn't there 2 chapters ago.
The drawing was also crisp and clean, without the haziness or jagged lines that sometimes make a manga hard to look at. The fight scenes were well planned out, with each one unique. It was easy to follow the path of the drawings, which I and any other manga fan greatly appreciates.

Character=9
Each character had their own personality and style about them. Yes, there was the obvious stereotypes: a guy who is mean on the outside, but is more trustoworthy than anyone: a girl who constantly worries about a guy, but then acts like she doesn't care: a comic relief who has a serious side etcetera etcetera. However, they still had some things about them to make them different from the norm.
Only reason I didn't give it a 10 is because I thought that some of the main characters should have had a little more background given to show why they were the way they are. So in all honesty, I would have given this category a 9.8, but you can't do that.

Enjoyment=10
I consider the level of enjoyment how well the plot turns out. Sort of like the story category. This particular manga was great.
There was just enough fighting to keep even the most violent person entertained, but there were still time for peace and quiet. Not meaning that there were tons of fillers. I can't even say there was a single one.
Each chapter had a reason behind it; either to add to the story developement or show something about a character you didnt know, or will need to know later. I don't remember a single chapter that didn't have reasons behind it, however small they were.

Overall=10
It is rare to find any form of entertainment that comes close to the other 4 categories' ratings. Even though I didn't give it a perfect score (which is impossible to me by the way), I still say this as the best manga I have ever read. Any fan of shonen manga, or manga in general, should add this to their list.