Definitely the most underrated of all the MCU films, Captain America: The First Avenger is a great war film done comic book style about a little guy making a big difference. Red Skull is one of the better villains simply because of how fantastic Hugo Weaving is. Peggy Carter is a great heroine and Hayley Atwell's performance is an absolute joy. Also, Tommy Lee Jones and Stanley Tucci light up the screen whenever they're on.
With strong action scenes, nicely developed characters, and seamless CGI, Captain America: The First Avenger is perfectly fine as a stand alone film. Shameful that so many audiences were just going out of obligation for The Avengers, because this movie has a lot to offer.