I think I've spoken of my love of 70's superhero books here before, particularly the era when a small group of writers (namely Steve Gerber, Denny O'Neil, and Steve Englehart) began to use the medium to explore various social issues. The height of these stories were Gerber's Howard the Duck, O'Neil's Green Arrow/Green Lantern, and Englehart's Captain America, now collected in the fourth volume of the Essential Captain America.
The stories in this collection are considered by many to be the highest point in the character's nearly 70 year career. This was the first time in which the Captain was portrayed as having different values than his country and this fundamental change to one of the most iconic pop culture characters out there made him something he never had been before, relevant. Combine that with a handful of other lasting additions to both the character and the Marvel Universe (the first appearances of Moonstone, Baron Zemo, and Roxxon as well as the Falcon's wings), and this book becomes a key piece of comics history.