The best manga that I have ever read. If you're looking for something with deep, intricate lore, then this is it. You have found the manga. I'll keep this review spoiler-free as to not ruin the many surprises you'll encounter throughout the series. TL;DR at the end, and the review as easy to follow as I can.

The Story (10/10)

The story of Attack on Titan takes place in a time where large humanoid creatures called "Titans", which resemble humans, but are 5, 10, and 15 metres tall, the remaining population of humans was forced inside 3 50-metre-tall walls: Wall Maria, Wall Rose and Wall Shina being the most inner wall. Their just want to eat humans,
having no intelligence whatsoever.
Our main protagonist is Eren Jeager, alongside Mikasa and Armin, quickly learn of the terror of the titans after a 60 metre tall titan appears and smashes the main gate protecting humanity from titans. From that day onwards, Eren swears he will do anything and everything to stop the titans, and to go beyond the walls.

The story may sound boring and basic, but as the plot progresses, there are many, many interesting things happening. Isayama creates different situations for his characters, and sees how they interact with eachother. The plot is very story and lore rich, with many secrets hidden, and foreshadowings, which are more appreciated when you read the manga after you have finished it entirely.

The Art (10/10)

The art at first may seem a bit more cartoonish, more "water-colour-y" if you get me. But as the series progresses and the themes of it change, so does the art style, and also considering this manga was started in 2009 and will end in less than a month on 9th of April, 2021, Isayama's art style has clearly evolved from something good at best, to amazing and may I say a masterpiece level, since this is supposed to reflect my point of view.

The Characters (10/10)

The characters start a bit basic, we are introduced to them, their personalities and their views and ideas, and as the show progresses, they evolve with the plot, changing sometimes for the best and sometimes for the worst. Attack on Titan is the only manga where I truly loved and still love the characters. I like more characters in Attack on Titan than I do in any 2 other random series combined. They are well-thought out and fleshed to feel like real humans, with flaws, moments of humour and horror, depression and happiness.

Enjoyment (10/10)

As you could have probably guessed, this series truly made me feel bad for characters, wonder what is going to happen and always want to read "just one more chapter" before going to bed. It's one of the most brilliant surprises I have had in my life. I went into this expecting just killing titans and cool ODM scenes, and whilst there is plenty of that, this manga is so much more that it is hard to explain without spoiling you. This plot is amazing. I want you to feel the same way I felt when I read the second to last chapter, that.. feeling that the series' end is right around the corner and wondering what is going to happen... because that feeling is the best feeling. And I feel like these emotions are present pretty much everywhere. You'll enjoy this series, either you're into big, story-rich plots or just fun-action scenes with interesting characters, it has everything.

TL;DR
Overall, Attack on Titan is a great series, with wayyy more to with than you would expect, and a story and lore so rich you would want to read it again for the first time just to feel those emotions again. With art that gets progressively better from "good enough" to "masterpiece" and interesting characters, and an enjoyment beyond anything out there. This truly does feel like that 1 in a billion manga series where everything just clicks and so well thought out. Well, of course it would be, considering Isayama worked on the whole plot 6 months before even drawing anything for the manga (yes, it's that deep and rich).

Truly a masterpiece and my best read ever. 10/10