Hundreds of years ago, horrifying creatures which resembled humans appeared. These mindless, towering giants, called "titans," proved to be an existential threat, as they preyed on whatever humans they could find in order to satisfy a seemingly unending appetite. Unable to effectively combat the titans, mankind was forced to barricade themselves within large walls surrounding what may very well be humanity's last safe haven in the world.
source: Shingeki no Kyojin Mangarock
About
Shingeki no Kyojin (Attack on Titan) is a Japanese manga series written and illustrated by Hajime Isayama. The series began on September 2009 and is still ongoing.
Plot
"That day, the human race remembered the terror of being dominated by them, and the shame of being held captive in a birdcage..." — Over 100 years ago, a natural predator of humanity appeared: the Titans, giant humanoid but mindless monsters whose sole purpose of existence seemed to be to devour humans. There was an insurmountable gap in power between them and mankind, and as a result, humanity was rapidly exterminated to the brink of extinction. The survivors responded by constructing three concentric walls: Wall Maria, Wall Rose and Wall Sina, which graced them with a century of peace. However, one day a Colossal Titan far larger than any other seen before breached the outer wall, allowing the smaller Titans to invade the human territory and forcing the survivors to retreat to the inner walls. Eren Jaeger, a boy whose mother was eaten during the invasion, vowed to wipe every last Titan off the face of the Earth, and joined the military determined to exact his revenge.
source: Attack on Titan Wiki
Thoughts
Art
I will be starting with the Artstyle of this manga first which was really bad when it first started. The characters looks very similar to each other which was kind of confusing. There's just too much dark lines and shading here and there, which I get was the atmosphere the mangaka was aiming for but I think it's just too much. Details aren't drawn very well even the action scenes seems to be lacking. Not that I'm being too critical with the artstyle of this manga, it just gives off a rough and amateurish feeling when I first started reading it. Even Isayama (the mangaka) admits his works were amateurish. However as he published more and more chapters, you will definitely see how his artstyle improved.
Story
Shingeki no Kyojin's strongest point is its solid and interesting storyline. Humankind on the brink of extinction is trapped within giant walls surrounded by giant eating monsters called the 'Titans'. Who wouldn't be interested with that? To top it all of, the world within SnK is set on a real-life setting (which means no magic, or supernatural beings), which immediately raises the following questions, 'Where did all this Titans came from?', 'Are Titans natural creatures or a result of a scientific experiment?', 'Who and why are the walls created?' With mysteries like that, you can't help yourself getting hooked with this manga.
What made Shingeki no Kyojin popular however, is not that. It is the brutality and the darkness of its anime adaptation. Good guys getting eaten out of nowhere, squeezed or shred in pieces. Titan's pick. It's like watching an anime version of Game of Thrones but with the side characters dying. You can't help feel bad but at the same time, is waiting for someone to die horribly because it's just what this show was meant to be.
Since Shingeki no Kyojin is a monthly published manga. Many people feel that the pacing of the story should be faster. If you are one of those people who can't wait for a month until the next chapter then Shingeki no Kyojin is not for you. The amount of details the mangaka has put on this manga to create a perfect world is also worth mentioning. He is known for his unconventional and risky moves. Just last year, Isayama(the mangaka) performed what I think is the riskiest move ever in the manga world when he completely shifted our view out of the world we are used to. For almost over a year, we are introduced with a lot of new characters. I for one, almost dropped this manga because of that. I mean, we are here for Eren, Mikasa and Armin's story not some random people out of whatever island they are in. However, when the two worlds finally intertwined that's when you'll see how focused and dedicated Isayama is to this perfect world he created. Currently, Shingeki no Kyojin has 101 Chapters and is still ongoing.