I don't have an answer any better, or even just as great as these two. They've pretty much said it all. An anime is great when it leaves you speechless or awed. The more the animation, acting and other components are on point, the better it will be, but
if it touches your heart somehow as well, then it must be a masterpiece. This is how I felt about Shin Sekai Yori. It's such a work of art, I don't know what to say about it because my words won't do it justice in my opinion. But I've also realized that for
every anime I deem a masterpiece, my feelings are different: for example, my feelings for Sailor Moon are related to my experiences growing with it as a child because it was my first and biggest inspiration as an artist, but I was just obsessed with it too.
When I re-watched it I was able to identify with so many other things, and I understood things I didn't even notice were there before, but I also learned something that today is very important to me. My love for Naruto is a lot different, because it communicates
to me that I can be a better person, have a higher self-esteem, and it also deepens my knowledge of certain things. And my feelings for Shin Sekai Yori has nothing to do with personal development, but it does make me question some personal things, and from
that I've established (or am establishing) new ways of thought. But for the most part, it's just art at its fullest potential.