One vote for : AoT.
I watched all anime that people called it "masterpiece", in top 10, etc, and they're rights. They have a lot of good points, good enough to acquire the title "masterpiece". Except AoT. Because of its story. Something of the story.
An example is Code Geass. Especially, R2. I know it has some dumb moments, but the good outweighs the bad. And its fascinating ending. An unpredictable ending.
All of the stories has its buildup.
But, AoT, you know. I won't spoil anything for you because I don't know who you are: an anime-only or a manga reader.
If you are a manga reader, you will know what I say. And if you like it, that's OK. But I don't.
If you are an anime-only, keep waiting. To the end of this anime. And you will have your own answer, about: is AoT a masterpiece?