Kuroshitsuji, or Black Butler, is a manga composed of one part serious historical fiction, one part supernatural horror, one part fanservice, and five parts uncut crack.
Twelve-year-old Ciel Phantomhive is the last of his line and the Earl of Phantomhive, following the death of his parents in the fire that claimed the family mansion. He lives alone but for his servants: one who doesn't do much of anything, three who fail at everything they attempt, and the titular butler, Sebastian. In his own words, Sebastian is "one devil of a butler."
Literally. Sebastian is a demon, contracted to Ciel to serve him until Ciel meets his goal: the death of all who conspired to kidnap him and murder his family. Until that time, Sebastian is happy to serve his master in any manner required, whether it be shining the tea service, hunting down murder suspects, or ever-so-politely eliminating all threats to Ciel...other than himself.
The art can be gorgeous—although it frequently descends into chibi—the relationship between Sebastian and Ciel is a beautiful study of loyalty and sadism in equal measures, and the plots careen wildly between deadly serious (like a Jack the Ripper arc) and dreadfully frivolous (pretty much anything to do with Ciel's cousin/fiancée, Elizabeth), and occasionally both at the same time.
While the first few storylines can be temporally wobbly (cell phones in Victorian England?), it settles down after a bit with exquisite attention to detail. If you don’t mind mental whiplash, this is definitely a series to try.