For me Kuroshitsuji is my most favourite anime ever! I can't stress enough how much I love it! It has everything I am looking for in an anime. Thick plot, dark feeling, awesome comedy moments that don't ruin it overall though, and so many amazing cast of characters.
The voice actors as well did their job fantastically, espesially Ono Daisuke, the seiyu of Sebastian, my personal fav, who also won best male seiyu of the year for this role. And I think we all can see why in the anime.
The music is amazing, a FANTASTIC opening theme and a dreamy, magical second ending theme. Also the background music was equally amazing. The art is stellar!
And I could talk about it for ages. My personal favourite episodes must be the first stroy arc, until episode 5 or sth.. and from episodes 17 until the end.