Incredible, I honestly have to say that this could be the best anime ever due to its development and plot. Shingeki No Kyojin has one of the best character developments and plot in history (note that this series does not have fill), he even the secondary and third characters have a great importance in the story, but also has that sense of terror, action and drama that by the way is incredibly well done, makes you live the situation that the characters go through and transmit their emotions to you in a very human way. It has an excellent background story that is simply memorable.
Every decision they make is of minor importance to the series and should not be missed, their deaths and the surprise of knowing who are the real antagonists add a magical touch
The first season has a slight start, it ends up being a spectacular season, it is not the best but it is an amazing season with an incredible animation and plot, here you will be introduced to the first antagonist of the series. 9.4/10
The second season I think was epic, this is when you start to see the faces of the real villains (get ready for Senshi Tmp 2 Ep 6 which according to me is the best of the second season, an overwhelming chapter) Shingeki no Kyojin explains it to you perfectly without leaving gaps in the plot, an incredible season. 9.6/10
The third season is truly phenomenal, has a good start and lets you know the problems you will have in a society by not agreeing with something or the difficult decisions you must make to ensure your survival. After this, too many things are clarified without detours, the chapters that stood out most were Perfect Game (Tmp 3, Ep 16), Hero (Tmp 3, Ep 17), Midnight Sun (Tmp 3, Ep 18) and That Day (Tmp 3, Ep 20) (That day has no action or features our main characters, but this chapter amazes me, its development is spectacular and the way they explain everything is overwhelming, honestly, the best development chapter in the history of anime). 9.8/10
Season four is the start of the finale. A season practically perfect, in this season everything comes to light answering every question of the viewer, leaves no gaps in the plot and has a beautiful animation, besides that is incredibly shocking and demonstrates the incredible development in our protagonists, Eren has the best character development I think. For me the best chapters were in my opinion Declaration of War (Tmp 4, Ep 5) (Declaration of War is one of the most impactful chapters of development), The War Hammer Titan (Tmp 4, Ep 6), Assault (Tmp 4, Ep 7), Assassin's Bullet (Tmp 4, Ep 7) (Assassin's Bullet left me speechless...) and Above and Below (Tmp 4, Ep 16), has not yet finished this season, but still for me it is spectacular. 9.9/ 10 This season is the best.
In my opinion your average would be 9.3/10. Sincerely from my point of view, the best anime ever, a work of art from post-apocalyptic fiction.