These are things I consider masterpieces. There's a very particular thing I mean by a masterpiece that's difficult to put into words. First off, a masterpiece must be cohesive and "whole". It must stand on its own as a work of art. It must have a consistent aesthetic, and it must be a superlative instance of that aesthetic.
Secondly, whether something is a masterpiece or not is not necessarily tied to whether it is one of my favourite works of art or not. I think this makes sense based on my definition of masterpiece, because something that's my favourite will likely be especially relevant to me and my life, whereas a masterpiece stands on its own.