Two quiet anime: Non Non Biyori, Ookami to Koushinryou.
My definition of anime masterpiece would be something that really blew my mind, got me thinking/"feeling", and had very few flaws (art/production/animation/storyline flaws/poor character development/overharem...just a few major flaws I see in today's anime).To be honest, you should either wait to have seen more anime before grading a ten and optimally even grading them at all, because at the moment, 1/6th of your list are tagged as a masterpieces, and it's bias by the fact that you haven't seen a lot of animes, so since those are new to you, you have the impression that they are incredible in every aspect, but if you would watch more animes, you would eventually see that it was kind of a illusion covered by the fact that it was something you've never seen.
I gave these anime a score of 10/10 on myanimelist:
Hunter x Hunter
Darker than Black: Kuro no Keiyakusha
Chuunibyou demo Koi ga Shitai!
Nagi no Asukara
Fate Stay Night: UBW
Grisaia no Meikyuu: Caprice no Mayu 0