I think my favorite part about this manga is the world building, and how well thought out the plot is. It's evident that Kori Yamazaki (who is a she, by the way) did a lot of research, and her effort shows in how detailed her world is. I love how she interprets European folklore and myths, as well as how that all fits with the modern world. 

I can't say I have a favorite character, but I think two of my favorite are Ruth and Cartaphilus. The latter is probably a bit of a surprise to people, since he's done such cruel and harsh things to people, all for his own selfish sake. But it was a hinted at somewhat early on that he isn't just a one dimensional villain, and later chapters prove that to be correct. I think the level of empathy readers will have for him will definitely vary, but just as far as a villain go, it's not hard to see how he became who he is because of the curse he carries. Ruth is a character I love because while he looks human most of the time, he is still at heart, a dog. He's protective of Chise and loves her unconditionally, but he's always not ruled by human morality, and is willing to look the other way if the death of one person means his master's survival and well being.

My favorite chapter in the manga so far is the star-crossed love between Joel and the Leanansidhe. We were introduced to them a bit early on, initially just as a point of introducing another mythological being in Chise's magical world, but the conclusion to Joel and Leanansidhe's story turned out to be heart-wrenchingly bittersweet.