If you like mangas with a good balance of action, drama, gore, politics, psychology and realism, then you're in for a treat. Shingeki no Kyojin's very dark mood is somehow balanced with it's very shounen-like main character. The art isn't that refined but this seem to add to the mood of the story. It matches the sad and tragic feel of the manga. The feeling I had while reading this manga is similar to watching World War II movies and series.
Synopsis: Several hundred years ago, humans were nearly exterminated by giants. Giants are typically several stories tall, seem to have no intelligence, devour human beings and, worst of all, seem to do it for the pleasure rather than as a food source. A small percentage of humanity survived by walling themselves in a city protected by extremely high walls, even taller than the biggest of giants. Flash forward to the present and the city has not seen a giant in over 100 years. Teenage boy Elen and his foster sister Mikasa witness something horrific as the city walls are destroyed by a super giant that appears out of thin air. As the smaller giants flood the city, the two kids watch in horror as their mother is eaten alive. Elen vows that he will murder every single giant and take revenge for all of mankind. Nominated for the 4th Manga Taisho Awards (2011).
This manga won the Kodansha Manga Award this year.
Here's the link: http://www.mangareader.net/shingeki-no-kyojin
I hope you enjoy it!