The plot, artwork, characters, fighting styles, weapons... I think all of them were unique and inspiring for the other samurai series. Let's be honest, even their ways of dying
(Makie, Shira, Anotsu etc.) were original compared to other manga. It wasn't possible to predict the outcome of the battles until the very last moment. I'm quite satisfied with the realistic ending which doesn't force its way just to please the readers with some crappy happy epilogue.
What I found admirable the most is that the characters had depth and sensible purposes and reasons in order to fight with each other, and all of them was distinguishable from each other with their unique appereances. I believe the only truly villain was Shira after all, ahh he was such an original psycopath. He wasn't someone who could be intimidated, compromised or negotiated under no circumstances.
Eventually I think the strongest characters in order would be as follows: Makie, Habaki, Anotsu, Shira, Giichi. (Since it's not possible to analyse Manji's power objectively due to his immortality, I exclude him from the ranking.)