8. Captain America
Of all the sub-series within the Marvel Cinematic Universe, the Captain America trilogy is the most consistent, the most nuanced, and the most integral to the franchise as a whole. Each one of these films has had a major impact upon the future of the universe, introducing key plot elements and shaking up the status quo right before the next Avengers adventure takes place.
What's also most fascinating about the trilogy is how they have evolved. The First Avenger was more of a simple superhero adventure story in the vein of The Rocketeer or The Phantom, but once plunged into modern day Cap evolved into a different kind of hero. The Winter Soldier took Cap to a world of political intrigue and moral questioning, and then Civil War further upped the ante by pitting him against his allies.
But even through these evolutions, Chris Evans' portrayal of Steve Rogers is consistently enjoyable and stays optimistic even through the darker modern times. These movies are ultimately a better example of how to make a Superman movie than the actual modern Superman movies, managing to portray a selfless and nigh-indestructible character who is somehow both likeable and relatable.
It is currently unknown where Captain America will stand post-Infinity War, with there being a good chance a new hero will take the mantle in Phase 4. But either way, these three films together stand as a perfect microcosm of the idealist concept Captain America has always stood for.