Imo, there are many masterpieces. Every movie I rate >9.5 is a masterpiece for me.
1. a person's greatest piece of work, as in an art.
2. anything done with masterly skill: a masterpiece of improvisation.
3. a piece made by a person aspiring to the rank of master in a guild or other craft organization as a proof of competence.
Yes, there is no flawless movie, but flawless =/= masterpiece, so...
Cowboys and Aliens - 8/10. I was seriously impressed considering it manages to juggle so much, i.e. a story about both cowboys and aliens, a story about both cowboys and Indians, a story about a guy with amnesia and a whole load of supporting characters and still make it work. This is obviously due to influences of some great films on the different aspects of CaA, like Last of the Mohicans for the scene when they arrive at the Indian camp.