The "MUST WATCH" MCU Films

Actually if you don't know anything about this characters avengers would be a mess. Think about it. Why is that guy changing into a green monster, why is that one going around with a hammer?

So in order to understand what is happening you must either 1- watch every movie (including origin ones) or 2- have some kind of previous knowledge of the characters, wich lets be honest everybody knows where they come from.

You could argue you really don't have to know the backstory if you just accept the characters like for instance Darth Vader, but even he gets an explanation (fallen jedi etc) but not why he has the mask etc. You can get away with one or two major characters like that but not an ensemble.

And I know of a lot of people, real people who, if not for the previous ones would feel lost in avengers. Not Fans not comic readers but normal, i don't care about super-heroes people that like MCU because of just that, they never feel that they have to know anything before seeing Iron-man (1 and 2), thor, first avenger or Incredible Hulk, and if they feel lost seeing avengers they just have to pick up the one that tells them what happened to that character before the ensemble. 

Now if you are talking about people that already knows about where these people come from or generally accept things like super-heroes popping up without explanation point them in the direction of Justice league (jut kidding) i think the ones that reference Infinity stones\thanos are the way to go:

Skip all the first ones, including first avenger and iron-man. The tesseract is explained in avengers and Cap and Iron-man are pretty much the same guys from the end of their solo movies, as are all the rest of the avengers. This movie begins with the basic story that everybody knows intact for each of the characters.

So:

Avengers
Dark World
Guardians
Age of Ultron
Doctor Strange