No. 4: Captain America - The Winter Soldier

This was probably the movie that changed the MCU the most. With the destruction of SHEILD at the hands of HYDRA, and the use of Bucky Barnes (aka Winter Soldier) against Cap, nothing was ever the same.
In Age of Ultron the Avengers were literally alone. All they had was Stark's resources and a little help from Nick Fury and Maria Hill. It honestly took me a hand full of views to actually start fully appreciating this movie.
At first it felt like it might be first movie all over again, but now, after all the times I have seen it, I realize that it definitely deserves this spot high on this list. With its great script and character development, Captain America stands our from the crowd and remains one of Marvel's biggest accomplishments to date.
Oh, and the first Stephen Strange (Dr. Strange) easter egg helped too.