My top2 because FMA is top1.
The script leads us to a personal drama of each character. They really grow during the story.
All of them.
RK introduces us to several charismatic characters, each with their own past. Each one seeking their future. And every new moment puts them in trouble, they need to think (I learned a lot about screenwriting, from this manga, including a hallway/alley fight scene)!
The design changes over time, for me Shishio's phase the design was at its peak.
With this manga, I met the Shinsengumi, in about 3 pages. After that I googled. After that I fell in love with this group. In fact, I'm a fan of Shinsengumi (and I've even cosplayed as Shinsengimi Soldier).
The system of techniques, well explained. Character creation is excellent and you can read about it in the extras.
I literally could write so much more, this manga changed my life, I read it in 2001
10/10 to me