I've read a lot of smart manga, but this one is so well-thought-out, well-researched and intelligent that it's ridiculous. The art is simpler than I would like it to be but I've been literally stopping to think about how good this is after finishing every chapter.
The anime adaptation is one of the first examples I'd use to show someone how much this medium can accomplish, but this manga somehow got even more complex after the point the anime stopped at.
Nitorin's bravery is like Chiba's intelligence: less observant people might simply not notice it, but it's astounding in magnitude.
EDIT: What is ambiguous about Anna's and Nitorin's relationship? Anna's sexuality is very much ambiguous, but they clearly are, and have been, back together for a while now.