Berserk

Berserk is a fantasy horror seinen. It contains a lot of gore and explicit sexuality and even rape. This manga isn't suited for minors. Also this manga is rarely released. You can count with 4-6 chapters a year, so if you aren't a patient person you might consider missing this one out.

Berserk takes place in medieval times. It is the story of Guts traveling the world hunting for gruesome monsters on his quest for vengeance. Searching for the man who branded him, fighting against his cruel destiny.

Firstly I'm sorry for the short plot description but I wanted to avoid spoiling. It would be impossible to reveal the true story without telling about certain events, witch would damage the reading experience.
Guts starts out as a very unlikable character but once his tragic life is revealed his actions start to be painfully reasonable.
Berserk is the best fantasy story I've ever read, except maybe for Lord of the Rings. The author portrays strong feelings like friendship, love, admiration, desperation etc... extremely well.
The characters have a lot of depth and are well made. You'll learn to love some characters so much it will be painful to see them gone.

The artwork is the best i ever saw in any graphic novel. At the beginning it's not that impressive but it gains a lot of quality over time. Especially impressive is that the author doesn't have any assistants and draws everything by himself. These two facts are most likely responsible for the slow release pace of this manga, it's very well imaginable the author spends a whole week on certain panels.

The first appearance of berserk was 1990 and the manga is currently at 317 chapters and ongoing. This is going to be the lifework and legacy of the author Miura Kentaro. If you are not disturbed by its slow release and brutality, I give this manga a very high recommendation.