This is definitely one of the best anime I've ever seen. The story and characters are wonderfully well-developed. What I like most about this anime is that it doesn't just focus on the typical "hero's journey" that we all know, but rather focuses more on showing us a being without emotions and without origin who experiences a myriad of things in a rather dangerous world. I also like that the anime itself isn't afraid to tackle complex topics and censor certain things. This anime was, especially its first season, an unparalleled audiovisual and narrative marvel.
Its third season is currently airing, and it seems to be much more interesting than the show has been so far.
Good review.