I've never liked the term "masterpiece". It's loaded with different meanings that, if not clarified, confuse the hell out of any discussion about a film.

The term could mean, as it commonly does, that a film is in a "greatest" list somewhere. In that case, there's no arguing about it -- it's definitely in that list, and so it fits the bill. But wait! That list also includes 2001: A Space Odyssey and Shawshank Redemption, and those awed me to tears, while this film did nothing to me. Ah, so that term doesn't mean being included in a list; it actually means "an awesome film (to me)". But hold on a sec, while you may think those two films are awesome, I think they're shit.. and thus begins a typical internet contest for the last place in a thread. John Locke wrote about this problem in An Essay Concerning Human Understanding. Everyone should read it.

There are other meanings. For example, some people use the term to refer to a film that's representative of a film-maker, in that it deals with his/her typical themes, shows all his/her stylistic traits, etc. In that case, you must watch many of his/her works in order to see why it's a representative film. Watching just one wouldn't cut it, right?

Or it could mean the film had some artistic breakthrough. In that case, you must be familiar with the state of the art at the time this film was released, and compare it with other films that are contemporary with it to see why it's special.

Or it could mean the film demonstrates superior craftsmanship. This is the hard one, and there's no guarantee an exquisitely crafted film wouldn't bore the shit out of you (in fact, unfortunately, they often would). In this case, which part of the craft are you focusing on -- Script? Cinematography? Editing? Or what else?

So, what do you expect from a "masterpiece"?